Another +1 for the comments above. If you're using lots of AF zoom lenses, you might even find a katz-eye (or similar) a hinderance as the viewfinder gets very dark from f5.6 onwards (depending on the lens and the lighting of course).
I've very recently acquired a katz-eye for my K200D as I use almost exclusively fast manual primes (and one fast manual zoom). As a result I wasn't worried about optibrite, and am more than happy to accept that if I want a shot at f22 I'll have to compose it much closer to wide open, and then stop down before releasing the shutter.
I'm extremely pleased with it thus far, simply because it serves my purposes and has very obviously improved my ability to manually focus (especially in poor light).
If you MF, awesome. If you MF occasionally, perhaps less so