Yep, been using it for almost a year now. Hadn't heard of it before but I wanted to replace an old topfield pvr that kept rebooting and try HD at the same time. It also appealed to the geek in me!
We now have a backend tucked away in the loft and 2 front ends connecting into it. The backend is a server I put together from bits and has 1TB storage (on mirrored drives :-), 4 freesat feeds and 2 freeview feed. Frontends are Acer Revos - brilliant when running linux - cheap, small, quiet and capable. Ours are mounted on the back of the TV's and will handle HD (bbc hd, itv hd and also some bluray rips - 16gb files!) no problem. Connection via hdmi and s/pdif to surround sound. It works ok over wireless for most things but does need the cat6 for the bigger files / higher quality stuff. I believe you can run the frontend on xbox 360 amongst others.
I'd definitely recommend a separate backend if you have the space to hide it away somewhere. Be aware that some TV cards dont like to be shutdown / hibernated - my hauppauge nova-t 500 needs a hard power off or it doesnt work. A reboot / hibernate kills it, so putting it in a front end (which i tried originally) just didnt work unless we wanted that front end turned on all the time.
The Mrs uses it quite happily so once its up and running, its definitely 'family proof'. We control ours with a standard ir remote, the interface works well with it.
Advert flagging doesnt work with uk broadcasts. You may be able to make it work but I havent bothered.
Mythweb / mythmusic / mythgallery / mythgallery are all very useful plugins. I've had no problems with it playing all manner of video / codecs.
I did it with ubuntu 10.04 then installed stuff on top if it but you can get distros with it built in. There are also some plugins for the later versions which will connect it to iplayer, itv player, etc.
Theres a very good guide to installing at http://parker1.co.uk/mythtv_ubuntu.php
Get all the components (especially the tv card) working before installing myth!!
Enjoy!