For those who don't know anything about mythtv, it is a linux-based DVR, and much more because it can also manage all your music, photos, and videos. It is also starting to have good hooks into multimedia on the Internet.
Here are some thoughts about mythtv if you're considering setting up your own, based on my own experience. I'm using the latest mythbuntu 11.04, but the last few versions always seemed to have a glitch or two, but I need the later versions because of the better hardware support. Right now the main issue I have is with the remote occasionally quitting for no apparent reason.
1. Setup: Decide which set up you want: an all-in-one system, or split the back-end from the front-end? I think the all-in-one is somewhat simpler because you do not have to rely on networking to play back what you've recorded, and you may need a pretty fat pipe between the two (hardware is best), or your wifi better be pretty good. I didn't want more than one computer running all the time at my house, so it needed to be able to run both the mythtv back-end and the web server: with mythweb, you can also do stuff remotely (schedule shows to record, download/play your music/shows, ...).
2. Signal: If you have cable, find out from your cable company what signal you get. Over-the-air (like I have) is almost all digital (there are still a few very local very weak analog stations, but nothing worth recording), but I know that some cable companies are still analog, or could even have a mixture with some analog and some digital. I've only used mythtv with tuner cards, so in that case plug-in your feed (i.e. antenna or cable) directly into the mythtv tuner cards (and with a splitter you can also feed your TV separately).
If you have a separate mandatory (i.e. which you cannot bypass) tunerbox from the cable company, it's going to be much harder to control it from mythtv. I hear there is some way for mythtv to blast IR signals into your cable box to change channels for instance (much like a remote does). But then you surrender your cable box to mythtv, and if it's recording, you cannot watch anything else.
3. Hardware: Find quiet hardware (at least for the front-end if you split them, and keep the back-end in a different room). Slower hard disks, slower CPU, slower fans, smaller power supply all help with the noise. Check the mythtv web site for known working hardware (ask does your hardware works with linux?). I have an ASUS M3N78-VM motherboard with a dual core Athlon II processor running at 3GHz but consuming only 65W. This lets me get by with a 400W power supply (less power, less heat, less noise). I use the built-in nvidia display and sound. I bought a StreamZap remote that I knew worked with linux: I didn't want to configure one from scratch. I also decided not to bother with Blu-ray at this time: its hardware requirements are higher, and it's hardly worth it on a 27inch TV, like I have.
4. Video capture: If your cable is analog, get a hardware mpeg encoder to offload the encoding from your CPU. For digital, you don't need encoding since the signal already is. I have the Hauppauge 2250 with dual tuners: it wasn't well supported at first, but now it has kernel support for the digital tuners only (I'm not sure what it would take to support its analog tuners). It's very handy to have multiple tuners to resolve recording conflicts (if you're mainly recording PBS, you may be OK with one because they repeat most of their shows at night). You can have multiple cards (if you have enough PCI slots), so you can have any combination of analog and digital tuners if need be.
5. Video display: If you still have an old analog TV, check if it accepts anything beside an RF signal. If you plan to keep it, then you need to get some video card that can output the right type of signal. If your cable offers HD, it might be worth getting a new TV, or even a big computer monitor (although with the latter you cannot view TV except through mythtv) with the right type of input for your computer (e.g. mostly HDMI nowadays). I'd put the money in a new TV rather than a new video card (nvidia cards work well with linux): I suspect that the picture may be very disappointing on an old TV.
6. Sound: Almost any amplified computer speakers will do, and it's easy to upgrade separately. I still have some unresolved 5.1 or digital sound issues, but analog sound (like I'm using for now) is very satisfactory.
7. Yearly cost: You'll need a subscription to ScheduleDirect to get the TV listings: $20 per year.
It's definitely not quite plug-n-play yet, so decide if you have the time and inclination to resolve the small issues that you'll run into. Decide also if your roommates are willing to put up with it :-). It satisfies my tinkering urges, so I'm really happy with the end result. I think that the hardware I have will last for many more years, except for the hard drives which I'll upgrade in a year or two (I also use this box for backups, and I'll need a few more terabytes -- I do not backup the shows that I record with mythtv).