Rock Band is about making music with other people. It's about the joy of knowing that you are making the music.
The music selection is awesome. If you don't like the song then you probably won't care that much because you're lost in the gameplay. The singer might have problems though, but that ego could take a hit or two.
Enough terrible jokes, time to get down to the nitty-gritty of the game, the instruments. The guitar works well once you get a hang of it. (It feels tighter and more comfortable than the GH guitars, but works the same way.) The drums are a god-send, you'll have fun playing these regardless if you do well or not. The foot pedal you have to use gets annoying on long songs but it's worth it in the long run. The singing is touchy, and the weak point of the game for sure. If you have a band, you'll probably have the same singer for every song, and in Rock Band's Band World Tour mode, you have to sing every song. Girl songs and song sung by high-pitched men are extremely hard and sometimes you just can't hit a pitch.
The career mode for single player works almost exactly like the Guitar Hero games but the selling point for this is the character customization. It's amazing, you have so many costume and instrument choices that you'll be editing and messing around with your avatar's appearance for quite some time. The tattoo system is amazingly involved, and this again can consume a lot of time.
Where the game truly shines is the co-operative Band World Tour mode that I mention earlier. It once again takes you to different cities and venues but the cool part comes in with the extra things it lets you do. It lets you put together sets of songs to play if you don't like the songs it offers you, and if you don't care it gives you a set of two songs to jam to. After you play a gig you get the regular star rating and you get more fans. The better the performance the more fans, more fans equals more money and more venues, more money and venues starts the entire process over again.
If you have the choice between GH games and Rock Band, buy Rock Band. The price tag is steep but eventually they will sell the peripherals and game separate so you can pick and choose.