Terrible as in really only allow you to use it as a VCR. I have no idea how stable the Windows stuff is as I've never used it. I'm sure if you're going to use MCE then that won't be a problem. However from my understanding if the card doesn't say it's compatible with MCE then you'll have a hard time making it work.
They should work happily alongside other cards, although they interfere with my ordinary WinTV Go card tuner meaning I get interference on my analogue recordings.
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Cheers,
Andy M