Amazing game ruined slightly buy the crappy guitar
Firstly it is incredibily addictive especially once you start getting to the harder difficulties. You'll always be getting better and therefore always improving your high scores which just makes you want to play more.
I have put difficulty as "Hard" because although at the beginning it has a good difficulty curve and you can pick it up after an hour or so, the transition from medium to hard and then onto the later hard songs is anything but smooth. You may well find yourself stuck on a single tier, or even song for weeks.
One of the things that makes this game great is that you can listen to your favourite songs while you play it! The problem with this is you may find the game less enjoyable if you don't like any of the songs. They are mostly rock song with guitar solos (well it is GUITAR hero) so if you are into rock you will love it, if not, you may not like it. Having said that I wasn't into rock music at all before I got it and it introduced me to a whole new genre of music which I loved.
The main exceedingly annoying thing in this game is the crappy guitar. If you plan to play longer than it will take you to get to hard you may want to get the GH2 guitar instead. After that the buttons start to get unresponsive when pressed, the strum bar constantly overstrums. It gets very annoying when you know YOU FC'd (Full Combo, all notes hit without breaking streak) that song but the guitar overstrummed half way through or it didn't register a button press.
Overall this is a great game, very rewarding but if you plan on playing longer than a couple of months, get the GH2 or GH: WT guitar.