It's been six months since I dumped my $100/month satellite tv subscription in favour of a UHF antenna connected to a PC running Linux and MythTV and I'm pleased to announce that I have absolutely no regrets about the decision at all.
When I started talking about doing it, one of my co-workers cautioned, "You're going to end up being the sysadmin for your tv, do you really want that?" Honestly, it was one of the things I feared would happen. I worried that a package upgrade would take it out, or that MythTV would occasionally take a vacation and require manual intervention, but none of that has happened.
The setup has been rock solid, required less hands-on maintenance and reboots than the dedicated PVR hardware it replaced, and has survived numerous package upgrades and even a full blown distro upgrade to Ubuntu 12.04 without missing a beat. Having a computer connected to your TV has other advantages too. You can play games with MythGame, browse the web, or even play some big-screen online poker from the comfort of the couch.
If Linux could play Netflix, it would be perfect, as it's the only thing MythTV currently lacks that we use regularly to consume media