I use both Nik and Onone plug in Suite. Love both programs. Nik is much quicker and the tonal contrast is great for wildlife and landscapes. The Ononse suite is excellent, but I have not been able to use Onone plug Suite 5 in since upgrading to CS5 - It appears to be incompatable and have not received any response from their support in 3 weeks after repeated request. I hope they get it fixed soon.
It does work on CS5 MAc 64 bits, at least on mine.
They changed the way the plug-ins are launched, they don't show up in the filter menu but have their own dedicated window. I couldn't find it at first, but there is a FAQ on theit site that explains how to make it magically appear as a panel. Not sure to like that part as the design of their panel doesn't really fit that nicely (that's probably on purpose to make it harder for users to forget about them). Anyway, not a big issue.
In terms of content, Genuine fractal Pro is indeed good, especially for those doing canvas prints like myself. I also like Focalpoint 2.0 now that it finally seems to work per the specs in terms of transition between sharp and blurred areas.
Mask Pro is also pretty good, I have found it to be a good compromise bwteen ease of use and power relative to some other selection plug-ins. I don't see as much value in the other plug-is in the suite.
Recently I end up using plug-ins from topaz labs. They have progressed significantly thanks to the move to 64 bits and are now pretty stable and usable on large images.
Cheers,
Bernard