Plastic Renew works great, but it's LOTS of work! If you want to do a real good job, you'll be sanding for quite some time. I just finished reworking the plastics on my 03 KFX400. I am very happy with the results, but it took me a few nights of wet-sanding. I got all but the deepest scratchs out (only because I didn't want to sand forever!). When you're done the plastics shine nicer than new!
I also used a heat gun to remove white (on a kawi they're yellow...) marks. It helps alot but didn't totally remove them. If you do this, do it before the plastic renew, because if you get that 'sheen' from heating your plastics a little to quickly, the wetsanding will remove it anyways.
I did all this so I could trade in the KFX for a TRX, which I'm picking up in a couple of weeks! The dealer gave me top dollar for the KFX because it looked so good (= easy to sell for them), so it's definitely worth it!