The very first discs Ritek made weren't so good and had a bad reputation. But these days Ritek discs seem to be pretty decent quality all around, particularly the name-brand varieties such as TDK and Maxell.
As for Plextor and Ritek, the media compatibility thread shows Plextor doing fine with Ritek media.
The Ritek media I've been getting just lately has been extremely good -- I've been getting C1 ave/max 0.1/8 consistently burning at 52x on some BULK brand Riteks here. You can't argue with results like that......
Even the worst Ritek media on the worst writers seems to be coming out at roughly ave/max 1.5/20, which is still far from being a bad result. TBH if these discs last as well as they write, then Ritek have made the move from "decent cheap brand" to a quality manufacturer to rival the Japanese companies.
My experience with Ritek is limited, but essentially the same as jase's. The TDK-branded 40x disks are the best media I've tried- significantly better than the sample cfitz showed @48x. Of course TY's are next to impossible to find in Germany so I can't comment on that front.