Waste of money? Or are they any good? What genre would they mainly be played in? Any difference between a regular 6-string guitar (besides the number of strings)?
This post may contain my opinion and/or inaccurate information.

Current Rig:
2006 PRS CE-24
Mesa/Boogie Mark V
Voltage S212 w/ V30's
Strymon Timeline
CMATMods Signa Drive
TC Electronics Corona & Hall of Fame
I personally love my 12-string, it's good for when you need to fill in some gaps and a regular 6-string won't do it for you. get a cheaper one though because they don't tend to last long with such high tension on the neck. just make sure you play one first too, it's almost like a completely different instrument. I felt like an ass when I first tried to play one and I could barely play the correct string courses without hitting others.
The guitar forums are more suitable for guitar questions.
And no, Guitar Hero will not help. Even on expert. Really.