I would say yes, get it. unless, of course, you won't be able to make the rent/mortgage payment this month because of it. The game has some problems, but overall, it is a good game. The song choices are questionable, but once you actually play the game, you can kinda see why they chose these particular songs. I too am having the size issues, as the grip was designed for people with much smaller hands than me.
As for replayability, yes, it has it in several areas:
1. This game can and will easily be hacked to add unlimited songs in the future.
2. The multiplayer is pretty fun as well with a lot of power-ups and attacks that make it worthwhile.
3. A sequel to this game is already in the works and should be out around Christmas time, so you won't need to buy the guitar grip hardware again.
4. It is just a neat experience playing Guitar Hero on your DS. They have done a pretty good job or representing the GH brand.