I played the first one at a friend's earlier in the year and absolutely loved it.
Said friend's PS2 recently took a dump, and instead of replacing the system, he decided to just get rid of everything, so he sold me his copy of GH1 (along with the black guitar controller) for a song.
So I thought I'd try my hand at customizing the guitar. Cracked it open the other day, and put it back together as soon as I realized that--hey--I didn't know the first thing about customizing up a plastic guitar. Plus, I don't know what kind of design I want in the first place. So I'll think about that, and when I decide on something, maybe I can hit up one of those guys from the GH forum I linked to earlier, to do it for me.
Anyway, I went back and played the first game some, to tackle all the songs that were giving me fits the first time through, and I wound up breezing through pretty much everything except for the last few songs on Expert. So it's either that I've gotten a lot better since then, or that GH2 is much more difficult than the first game.
I think it's the latter.
Lot of fun, though. While the second is technically the "better" game, in nearly every aspect, I think I still have more fun playing the first one, for some reason.
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And in other GH news, the packaging for the 360 version of GH2 was recently unveiled:

Looks pretty spiffy. And the rumors going 'round are that the "additional songs and downloadable content" will be the songs from the first game, which would be pretty cool. It would also mean that there'd be very little reason to keep the PS2 version, if you have a 360. . .