It works, though obviously you can't expect miracles from HDV/ DV footage due the block artifacts. You will need a lot more strokes to tell the tool how to trace the edges.
Regarding DV footage:
People keying DV footage have learned lots of tricks for blurring the blue channel or otherwise getting around the fact that the blue channel is where the blocks and noise tend to be. You can use some of those same tricks. For example, you can use one of the Channel effects, like Shift Channels, to effectively ignore the blue channel in a copy of the layer, and then create your matte with that layer.
I just finished cutting a feature for TV that I also shot and supervised the FFX. There were a fist full of FX shots that needed Roto. The project originated on XD cam. When I first began working on the roto work I kept the footage in it's original form. IOW, I was editing original footage. The Roto Brush worked fairly well on many of the shots but was a real pain on others.
To prepare the project for color grading and the final effects work I spent about a day researching the best workflow to maintain quality and give me the most flexibility. It turned out that the best option was to transcode all of the footage used in the project to a YUV I frame codec. Because of storage requirements projected to be in excess of 15TB for Black Magic 10bit I opted for ProRez444. I revisited some of the shots that gave me trouble and the difference was astounding.
Further research revealed that any time you have highly compressed original footage you're better off to transcode the footage into a more production friendly format for processing. I knew this from years of working with DV footage but the amazing quality of P2, XDCam and even HDV footage made me forget that compressed formats are never the best option for processing.
So here's my advice: If you're doing anything more than straight cuts it's always best to Transcode to a codec that works in YUV color (not RGB) with at least 4:2:2 color space. Avoid GOP codecs for every step of the process right down to final output, and manage your gamma settings with color management. You will even find that a straight cut sequence that's been transcoded to YUV then compressed to H264 or sent to DVD will look much better than compressing your original footage.
One final note. We're now in the process of archiving the feature. I have decided to keep the camera original footage and only a ProRez render of the final cut because of the film's budget. If we have to revisit the project it will take a day to prep all of the original footage for editing. If the budget was bigger I would not hesitate to transcode all of the original footage and dump the camera original because when you are working with compressed footage the original is not the best you can do.
Difficulty in processing roto brush edits. It takes to long to get to the next frame to edit as needed