I'm lovin on IIDX gold. I've been playing the songs in order, starting with level 5 L7's. I've passed all those, and am moving through level 6's. I've come across one tougher song that I think I may have trouble in [forgot the name]. Hopefully there isn't too much of a wall for me though..
I consider IIDX games much more enjoyable/replayable than ANY guitar hero I've owned/played, and i feel Rock band is mostly a party game, albeit fun. Guitar hero, once you've beaten Devil Went Down to Georgia on expert, eh.
I've replayed it for funsies, upping my high scores, etc. Or to show friends the game. Other than that I never really touch them. Now IIDX on the other hand, I'll be working on for quite a while. It's a completely different skill-set than GH, DDR, SM, etc. I love it.