You can convert a small fridge into a kegerator for "about" the same price as a retail unit, but then you only get the two kegs.
If you went with a small freezer, and made a keezer, then you have spent about the same, and you can usually fit a few more kegs in there. If that matters to you.
Personally, I'd build one myself. I like building things. I'd also shop around for a good used unit and save some $$. Try craigslist.
I think the only reason for buying a retail kegerator is to not have to build one. The conversion has to be very simple if you get one with sanke tap.