MythTV is a bear to install and setup but it is well worth the hassle. I chose to use the "Add to Ubuntu" option of Mythbuntu on the frontend and backend. I tried the full Mythbuntu distribution but didn't like that as well as the "Add to Ubuntu" option.
I'm using my Dell M140 laptop as the front end. I plan to look for a suitable front end in a few weeks. I want something small, quiet, and compatible with my Samsung TVs (models LN-T4661F and LN-32A330J1D). I have a single Hauppauge HVR-1600 dual tuner video capture card in the backend. The analog tuner is connected to my Comcast cable TV service and the digital tuner is receiving over-the-air ATSC HD. I've used my PS3 as a front end too using the UPnP option. The PS3 works well as a frontend but it is not as nice as the Myth frontend interface.
The Myth TV configuration options and variations are many. It is overwhelming at first. Getting the capture card recognized and setup is tricky. Getting the frontend to talk with the backend took some time for me to figure out. All and all it is just a matter of getting your hands dirty with allot of trial and error. After hours and hours of reading and configuration it all starts to click. Wireshark the packet sniffer was helpful in understanding the communications between the frontend and backend.
Give MythTV a try. Tivo, your days may be numbered in my house.