I have one.
Well, what do you call "using"...
I use it as my display card with XFree86 and the 4496 drivers on a 19" CRT.
I have experimented with TV-OUT. It sucks.
The black border around the picture (underscan, which should be remedied with an overscan option in the config file as claimed by nVidia) cannot be removed. From other reports it appears that an earlier driver did not have this problem.
The TV-OUT cannot reasonably co-operate with the main screen. With cloning it can only run with the same resolution as the main screen, and the repertoire of resolutions on the TV-OUT is severely limited. Not usable with a 19" CRT.
The twin-view option disables the video overlay hardware so that cannot be reasonably used either.
The only option is "two X screens" but even then the video overlay does not work on the TV screen! I still have to see an acceptable video playback on the TV while the main CRT is active.
Video-IN and TV-IN: do not work at all.
The nVidia driver does not support video in, yet it blocks the loading of rivatv (by taking the I2C buses). It should either support video in, or it should make available an interface for other software to access the I2C while nvidia.o is loaded. No reaction from nVidia on this matter.
Besides that, the video encoder on my Peronal Cinema, a Philips SAA7174, is not yet supported in rivatv. I have asked Philips about a datasheet (which is not yet on their datasheet site) but no response.
The remote control is working OK.
I have submitted patches (and they have been accepted) to maintainers of "ATI-remote" and "Lirc" to support it in those drivers. The remote is similar to the X10 remote and also sold with ATI cards.
I hope this answers your question (and that the answer is what you like to hear)