I seriously recommend building one. My main argument is this: It's not like computers are going away, so learning how to build one is a skill that will last you a life time.
Your current computer will eventually become outdated, you're going to need a new computer in a few years anyway. So every time you build a new computer, you'll be saving yourself several hundred dollars each time.
You can use parts from your current computer: First, your computer case is probably re-useable, that's at least $50. Second, your hard drive is also re-useable... If it's a SATA, perfect. If it's a slightly older SATA, it might be slower, but you can use it to store data that you don't access/need everyday (back up photos). Also, your optical drive (CD/DVD burner). It might be slow in burning cd/dvd's, but if that optical drive is less than a decade in age, it will read fast enough to let you install the operating system. New optical drives are at least $30..
Yes, you save a little here and there by re-using computer parts. But you use them til they break. So, you space out the replacement cost.
Another point... it's really not all that complicated. most of the plug/jacks are designed in a way that you can't really plug things in the wrong place. Most hardware are backwards compatible anyway.
after you build a few pc's, you'll have eventually have enough left over parts. I've used left over parts to build computer a garage-computer (i'm acutally on it right now). I dont need to run into the house, and use my greasy hands on my "good" computer. Perhaps you can use left over parts to build a computer for kids? media-center computer?
Plenty of info on how to build a pc. google is your friend.
I bet a lot of diy-builders will attest to this: you walk by a display of computers at electronic stores or costco, and you think... gaaad, i can build a rig with higher spec for half that price.
that, alone, is worth it to me.