Its not really a rip-off - unless you say guitar hero was a rip-off of dancing games (Hit different things in time with music and falling symbols).
Some of the fan-made stuff is quite amazing too ("This week I've mostly been playing guitar" - Tommi Inkila is better than half the songs on GHIII - and the fan remake of Canon Rock is phenominal, Im surprised it's not a DLC addition for GHIII)
It's also a quite different to GH in some ways esp. if you don't play using a GH controller, though some of the songs are quite hard without it. For some reason too, I always felt like I wanted to beat my highest score, but in GHIII I'd usually play a song and then not return to more than 3-4. The difficulty increases seem a bit fairer, whereas in GH there was a HUGE gap between each of them.
I think it just needs a few more original songs that are just as polished as the ones FoF already has.