If you want to get a good Pop'n experience, buy an Arcade Style Controller from DJDAO
Welcome To DJDAO Bemania Arcade Style Controller Website
The thread for DJDAO controllers are somewhere in the Pop'n Music section. I think it's a sticky.
Guitar Hero is a good way to start out with rhythm games because it has a steady learning curve, but for learning to actually play along with a rhythm not OMG NOTES, I would only recommend Guitar Hero 2 (PS2 or 360, 360 version has more songs and downloadable songs) and Rock Band/Rock Band 2.
Other fun rhythm games that are best played with special controllers are:
Dance Dance Revolution/In The Groove
Pump It Up
Beatmania I/IIDX
Taiko No Tatsujin
Guitar Freaks
and Drummania