Totally worth picking up, even if you own the PS2 version already.
Although this game is basically the same as the PS2 release there's enough to feel like it's brand new. There are the 10 new songs in this game and the set list is rearranged quite a bit. Just this alone makes you feel like your playing a different game, kinda funny the way that happens. The 10 new songs are actually all pretty good. The 360 is also a great system for picking up a game like this because of the great achievement points that can be earned. Any skilled GH gamer can rack up the points super quick. But there are still a few that will provide some good challenge, perticularly the 500k and 1000 note streak achievements. There are only a few songs these are even possible on, hopefully your good at one of them.
There are of course downloadable GH1 songs to in case you only have a 360 and missed the first game. Of course, they are over-priced (I mean, why shouldn't they be right?) and not packaged in any logical order at all, but of course if you really need a few songs it shouldn't be to bad on you to pick some up.
There's not much to say about this game that hasn't been said about the PS2 version, since it's pretty much the same thing besides what I said above, so if you can definitely check this out and to all those with a 360 and PS2 you'll want to upgrade yourself no doubt.