Well either way, if some spam is getting blocked then it's pretty clear that Razor is basically "working" (you can check the razor logs in ~/.razor/ for more information). As to whether Razor is an effective spam filter, I don't have enough experience with it to comment one way or another. Personally, I'm not a fan.... and the 90%ish numbers sound to high to me.
But realistically, 50% isn't too bad (especially considering that our UBE blocks are probably blocking a good chunk of spam as well) -- and given the amount of spam that most people receive, 50% is a big improvement.
My problem with Razor is more the opposite - namely that people seem to report all sorts of non-spam things as spam. This is the reason that I've had Razor scored way down in my SpamAssassin checks for a long time, and one of the reasons I wouldn't use it to block spam.
Personally, I vote for an adaptive filter like bogofilter, or else a recent version of SpamAssassin. SpamAssassin 2.60 is doing a really good job for me. I'm requiring 6.0 hits to be spam and 8 hits to get dumped in my spam filter -- and I'm getting virtually no false positives and only about 1 spam every 2 days in my inbox.