I already have Neat Video's plug-in, but was curious to see how the Red Giant filter compared. I compared them both on a really noisy clip (you can also use these to remove subtle noise, but particularly with video re-compressed and uploaded on the web, those subtle effects are often lost anyway.)
This quick test revealed similar results, but with a couple of important differences. Denoiser produced very good results by just adding the filter to the video. To get similar results with Neat, I had to go in and produce a profile for the video: to produce a profile you have to select part of the video frame that is - ideally - a single contiguous color region, the larger the better. Producing a profile isn't too difficult and doesn't take long at all; providing you have a good part of the scene you can select. But if I just applied the filter, Neat Video doesn't do nearly as good a job, so for out of the box ease-of-use, Denoiser seems to be the winner.
Both filters let you manually adjust different parameters, few of which I fully understand (Neat options include: Temporal Filter radius, filter threshold, Mix. Denoiser options include: Noise Reduction, Motion Estimation, Enhancement, Sample Current Frame (with additional options), Mix)
Red Giant has a great video tutorial that takes you through these parameters. I tried experimenting with the parameters, but couldn't find I could get it much better than the defaults. I could get it different, but I wasn't necessarily sure I liked it better. At a certain point, these noise reduction algorithms seem to end up replacing the noise with different combinations of blur and patterning. You might prefer one setting over the other, but it's difficult to say which one is "right" or "better."
Frankly, I don't spend a lot of time tweaking Neat when I use it, so if I'm honest I probably would work the same way using Denoiser. Also, I find that I need to see the video itself, which means rendering each time after adjusting parameters, which brings us to performance; Speed wise, both take a bit of time to render, but on a 2.53 GHz Core 2 MacBook Pro, rendering this 8 second clip took Denoiser 9 minutes while Neat took 4.5 minutes, which is a significnat difference.
The price of both these filters is $99. There is a cheaper "Home" version of Neat available, but it only does resolutions up to DV size.
Red Giant's Denoiser filter seems to do as good a job as the Neat Video plug-in. It's slower, but it does this without having to create profiles. For that reason, I probably would recommend Denoiser slightly over Neat, but it's not enough of a difference for me to plan to buy Denoiser given I have Neat already.
Enlargements of part of the frame
Noise Reduction Comparison from Michael Murie on Vimeo.
Sample video. You probably want to download the original file from Vimeo to really see the differences.
Red Giant: Denoiser & Tutorial
Neat Video: Neat Video