I also be a MythTV User about a year.
Started on FC2 and have continually updated it.  I've reinstalled it about 7 times using Jared Wilson's howto, and once or twice on Debian.  I'm using an HP Evo D510 CMT with a RIVA TNT 2 video card, Hauppauge PVR-250, and onboard intel audio.
Problems I've had over this year:
Hardware:
*  Not a big enough hard drive (60 GB w/ a Windows install of 10GB.  Therefore, /video/store is only 40GB.  It is not a dedicated machine.  I use it as my desktop pc too.
*  Not a fast enough machine for clients.  I tested running a client mythfrontend, and it didn't work too well.  The Evo is a 1.7GHz P4:  Maybe it was my client or 100mbps network, but I hooked up a client pc running Debian, and it would skip frames, and the audio would get choppy.  I didn't do much troubleshooting, since I didn't use the client much
*  Live TV wouldn't work at one point until I disconnected power supply for 30 seconds, then booted.
*  Difficult to setup 2 audio cards.  This was machine specific, not Myth. I had the intel integrated and a SB Live card, and would have to alternate headphone jacks when switching between mplayer-plugin, and mythtv.  So I guess operator error.  I had edited the .conf file, but still couldn't get it working exactly how I wanted.

Software:
*Sometimes unable to delete recordings, and Myth will freeze.  After a reboot it works fine.  I think there may be a memory leak somewhere that may or may not be Myth related.
*Hard to get a client pc connected to run mythfrontend, and run locally as well.
*Trouble with audio capture on one or two installs.  I just reinstalled. 

Those are some of the bugs that I remember.  It may sound like a lot, but in all I think it is a great setup.  Some of the bugs may be due to my PC.  But it is sweet with the commercial autoskip.  I think the best thing to have would be >1TB of storage.  Once (if) Apple comes out with intel Mac Mini's, I'll get one to be a frontend in my entertainment center.  Right now I just run Myth on the HP in my den.