Unlike what frequently happens in the music industry, this follow-up is actually better than the original!
While the songs in general are a blast to play, the only downside I see is that I felt like I recognized fewer songs in Guitar Hero 2 than I did in Guitar Hero. I recognize nearly all of the artists in the main song list, however the songs themselves were completely new to me which made playing them very difficult. This could be by design since the developers wanted to increase the difficulty of the game anyway.
If you liked Guitar Hero, you will absolutely love Guitar Hero 2. In fact, you may even begin to think, "Why wasn't Guitar Hero like this??" If you do not own Guitar Hero or have only played it a couple times, I would recommend picking that up first since it is an easier game overall. Of course if you're willing to step up to the challenge, by all means pick up Guitar Hero 2.
I marked the Difficulty as Hard because while the game seems to be more lenient in terms of getting 5 out or 5 stars on songs, playing the songs is much more difficult. There have been a few times where I was absolutely certain that I was going to get at best 4 out of 5 stars on a song because I messed it up so badly, but I still ended up getting the 5. You will feel the difficulty more in your playing ability than in what the game tells you.
Overall, this is a great game that should not be missed by anybody that is a fan of both video games and rock music.