After you've looked at how to build one and punched the calculator on how much it will cost, I would also compare it to just buying a DVR.
Before I headed back to school (i.e. back when I had money) I got hooked on the DVR service available from my cable company with their digital package. One big drawback was that model had no way for me to archive things (i.e. copy to DVD). At school I moved into an apartment complex that included extended analog cable with the cost of rent. On a college student budget this made sense, but I sure missed that DVR. So I started looking around.
Toshiba makes two DVR models that will tend to work with most things you throw at them. Though I'm only going with the analog signal (no converter box at all) they include an infrared interface that will let the DVR switch channels on various brands of analog and digital cable converter box.
A second nifty feature is that these DVRs include an interface that works with the TV Guide interactive program guide. This is a "Tivo-like" onscreen program guide that is piggybacked on both over-the-air and cable signals across most of the country. It is FREE, and allows me to schedule recording a program up to 10 days in advance (as well as a "record every day or every week" scheduler).
These units also include a single-layer DVD recorder (only use DVD-RAM, DVD-RW, and DVD-R disks though) with which I can save programs that I like. Since they include optical sound out and progressive scan, these units are kind of a nice replacement for a standard DVD player.
The lesser model has a 160 GB hard drive and only supports 480P/i.
http://www.tacp.toshiba.com/dvr/product.asp?model=rd-xs35
The fancier version comes with a 250GB hard drive and upconverts DVDs to 720P/1080i, though I think it may still only record at 480.
http://www.tacp.toshiba.com/dvr/product.asp?model=rd-xs55
Because I'm a cheapskate, I ended up getting the 160GB model as a "refurbished special" from an online merchant that specializes in refurbished gear. All told, I spent $300 on mine and it has worked just fine for 1.5 years.
negatives? The whole menu system could be a bit more intuitive. For some operations I find myself going back to the manual. The cooling fan is also a little bit noisy, but I can't hear it when it's inside of my audio cabinet.
It may lack that satisfaction that computer tinkerers have in building their own unit, but I would suggest it is a serious possibility if the pre-built DVR does everything you want and if the cost of building one yourself starts to add up. A homemade DVR will probably have more flexibility about moving files back and forth from the hard drive, etc. I'm just saying it may be an option.