I've tried Noise Ninja, Neat Image and Noiseware, which I believe are the main ones.
The interface on Noise Ninja is my favorite, with the ability to mask areas, apply USM at the same time and apply different levelsof noise reduction in different areas. Also there are many Ricoh profiles available from their website.
I've tried some default comparisons between Noise Ninja and Noiseware, and IMO preferred Ninja as it kept more detail. However there's very little in it (pixel peeping required)
I believe Neat Image has similar fuctions to Ninja, however IMO the interface isn't as nice looking. I haven't directly compared the two's results yet, but will do so tonight.
Personally I think all three packages are very good and any differences are probably just down to different default settings, which of course are fully adjustable. Demostration versions are available, so I suggest downloading them and playing around for yourself before making a purchase.