Here's my personal evaluation:
Want a decent rig with good build quality that'll do most things well? It's probably cheaper to go with Dell or Gateway or one of the mainstream computer builders. I actually priced a mid-range system for a customer (I do PC troubleshooting on the side) if I were to build it versus just helping her with setup and getting it from Dell. Using really good components in the build, the Dell actually turned out slightly cheaper, and that wasn't even counting my fee to build the thing, so I advised her to go with that (plus you get the benefit of their warranty).
Now, if you're looking to build the ultimate system with *specific* parts, accessories, and features, you're looking at saving a LOT of money. Sure, there are companies out there that do it and do it very well (my last computer was from GamePC, and I love them, but they charge an arm and a leg). There's also Alienware, Falcon Northwest, Voodoo Extreme, etc... But they all charge *huge* premiums on the stuff, so you can save a lot doing it yourself. I still visit these sites to see what parts THEY use in their builds, and often time if you price a part there versus, say, Newegg, you'll find a 50%-100% markup on the custom builder's site.
In fact, that's why this time I'm going to be building my own dream PC, and I expect to save a few thousand doing it myself over what it would cost if I let GamePC or one of the other premium companies do it. At the same time, it allows me to build a much more specific machine than Dell will.
BUT, Dell's do make solid all-around gaming machines, and if you're looking for that, I really don't think you can go wrong. Just my two cents.