Of Rhythm games... Rock Band is King.
When it comes to a Rhythm game, Rock Band is the game to play. I have heard there are other games out there that did the 'band' type game already, but they never were big like Rock Band. I never played them, so while the concept isn't new, it is probably new to most people who will play this game.
Rock Band lets you take control of either the Lead Guitar, Bass, Drums, or Vocals in a virtual band. If you feel really up to it, the game was designed to allow you to do vocals and one of the others too if you feel like being multi-talented. The concept is pretty simple to see and understand on how the gameplay maps to the actions for each, so I won't get into the details of explaining that dynamic too much.
As you play the songs you earn points, the more notes or phrases you string together, the more points you will score. If you miss too many notes you get booted off the stage. I'm yet to see anyone score less than 2 Stars (out of 5) and still finish a song, though it might be possible, so for the most part you will get 3-5 stars (and on expert there is a Gold 5 star to obtain).
At the end of some songs are a 'big finish' where you either get to scream whatever you want or hit has many random notes on the guitar or drums to rack up points. You then have to finish it by playing the last few notes perfectly.
There are 4 modes to play on. Easy, Normal, Hard, and Expert. Each level increased the number of notes and how fast the notes come at you. In the case of the guitar parts, Easy only has 3 buttons, Normal 4, and Hard and Expert all 5. For vocals the difficulty effects how easy it is for you to stay on pitch and chain phrase combos.
Opposed to other rhythm games though, the modes are not progressive. For example, a song on Expert might be easier than a different song on Normal or Hard (example: Roxanne, by the Police - easy even on Expert; Afterlife, by Avenged Sevenfold - played on Hard is much more difficult than Roxanne on expert).
But before you start playing songs you have to make a character to play with. You can define Height, Weight (sort of), pick a face shape, hair, a 'behavior' (Like punk, rock, etc) that effects how they behave on stage, and of course give them a name.
As you play songs, you will earn cash, which you take your character to a store and buy all sorts of things to customize how they look. It is a fun diversion... but buying stuff and customizing your character only have aesthetic changes on the game. So it isn't necessary if you don't want to do it.
For some odd reason, RB doesn't let you create one person and have them play all the instruments. Instead you will have to create a person for Guitar/Bass, another for Vocals, and another for Drums. Each with their own money. Each that has to be customized independantly.
There are a few different modes to play though, Solo Career, Training/Practice, Tug of War, Score Mode, Band Multiplayer, and Band World Tour. Tug of War, Score mode, and Band Multiplayer can be played online, but Band World Tour cannot...
The Guitar, Drums and Vocals come with a Solo Career mode (why was Bass was left out?) that let people get use to the songs and learn the parts. There is a leader board for each track to compete against for high scores if you are the solo type of person.
Training/Practice lets you play any song you want, with any of the instruments, at 50% speed to Full Speed (at 10% increments). It lets you play the whole song, or just parts of it that you choose. It only records your total percentage of notes hit though, and doesn't show you combos or even how well you did in each section which is unfortunate. So you have to pay attention to the section name when you are struggling to find out where you need to improve.
Tug of War puts two people playing the same instrument in a battle against who can hit the most notes with the best combos. Each person will play certain sections of the song. Some parts of the song both will play. But for the most part one person does their thing, the next person does theirs, you both jam in a few spots. Whoever strings the most together will win.
Score mode is like tug of war, only both of you play the whole song. At the end the person who has the best score wins. This isn't as much fun as tug of war because you focus on the song the whole time, you don't have time to watch the other person really.
However, the meat of the game is the Band World Tour (BWT). Basically the premise is you and at least one other person (yep, you either need a buddy, or be able to play on instrument AND sing) take the stage and travel the world playing as many gigs as possible.
Up to 4 people can be in a band at a time, and only 2 have to be present to continue the tour. If you are planning to go it alone, I suggest creating the band with the Vocalist. That way you can switch from guitar/bass to drums whenever you want and still play with the same band (seeming the band creator always has to be present)
There is a main path that takes you through getting a manager, a PR firm, a 'sound guy', new transportation, etc... eventually leading up to being a Hall of Fame inductee and finally the 'Endless Setlist' (The 'Endless Setlist' is a gig containing every main track song in the game. That's right, all 58 songs in one sitting, ouch). You don't have to do every gig in every city to finish the main path of the BWT.
Along the road you will earn fans for good performances or if you tank a song fans will leave. There is a cap based on the difficulty you play at. That is to say, every time you 5 star a gig, you might not get more fans if you were playing on easy. You eventually have to play on medium to allow more fans to gather to your bands fold. You will have a maximum fan limit until you play on expert. Fans don't really mean anything in the game, but it is kind of fun to see how many fans you can accumulate.
Band Multiplayer is the equivalent of playing one song at a time with friends. You still get scored with the star rating and you can see who hit the best percentage of their part... along with some 'titles' given to different performances (like longest streak, most gutsy, etc). There isn't any completion to be had here though. Nothing tracks what you have or haven't played or your last score.
Rock Band comes with an amazing Downloadable Content feature. Every week more songs are added. Every song you download can show up on random setlists in the BWT or you can select them for the 'create your own setlist' gigs. They are also fun to play as simple multiplayer games. The DLC has some pretty good variety, all the way from Heavy Metal to Soft Rock. It is nice to have so many different options.
On the Xbox360 the achievements range from playing though the solo careers on every difficulty level to becoming an online tug of war champ... to finishing out the endless setlist on medium to expert. A lot of the points come from playing every gig in BWT for each city to become 'big'. For those only looking for quick points, this isn't the game to do it on.
Rock Band is a very big and long game. It works great for parties and for groups of people. There are some caveats to the game though:
1. If you are a master of Guitar Hero 3, Rock Band will be really easy in comparison when it comes to the guitar. Also, the RB guitar feels quite a bit different if you have learned on the GH guitars. And 'Big Finishes' are lame without the RB guitar finger taps.
2. If you don't have anyone else to play with, most of the game is in Band World Tour. If you don't like to do vocals, that eliminates over half of the game length for you right there or turns it into a grind you won't have much fun doing.
3. DLC can be viewed as 'expensive'. At typically $2 a song that's double than getting it from iTunes. With the up-front cost of the band kit to begin with this can be a pretty expensive game.
4. The instruments have been known to break or be faulty. I myself had a bad drum kit out of the box. BUT, the technical support was quick and will actually send you a replacement set before you send yours back. Shipping was free and quick. But it was kind of a downer to buy the game and wait 5 days before I could have a drummer.
5. The track-list caters to the 'older' crowd I believe, or at least to those well versed in famous Rock Titles from the 70's and beyond. There are more coming every day though and most of the songs will grow on you even if you don't know them initially, but maybe that was just me.
Myself speaking though I actually picked up GH3 after RB (and find GH3's difficulty annoying), love vocals, DLC and price wasn't a problem, I could deal with service, and the track-list is pretty darn good (only 3 or 4 songs I don't like to play). I would have liked Band World Tour to be online though.
So the short of it is this, if you haven't played it, you should. But you are not going to be able to rent it and enjoy it due to the peripheral requirement. So if you are going to try it out before you buy, you will need to find a friend that will invite you over and won't laugh at you as you learn the ropes.
It can be a little scary to pick up a plastic guitar mold, do karaoke, or try to play drums for the first time in front of someone who will do much better than you... but if they are nice people and worth keeping as friends ;) you should have a pretty darn good time.
Rock Bands positives outweigh any negative to be had in the package though... and with more songs coming every week. Add in the promise that all RB DLC will be compatible in Rock Band 2 (as well as the instruments) leaves little reason not to get this and get jamming, So I have to grade this above the curve as an absolutely excellent game.
PS: But for each caveat I mentioned above that you could not tolerate, minus 0.5 or so to find the 'true' score for you.