What I'm going to say has been very well-put already, but I'm feeling verbose!
This topic "touches me personally" (woo-woo...hubba-hubba!) because I just recently built my first, home-made system. Darn proud of it too!
It all hinges on your skill level, familiarity with computers, experience with software, and confidence level.
I had (still have, not using

) a Dell. It worked great, no probs. I bought it because of the warranty. I didn't want to have to worry that if something broke, I'd be screwed. 18 months ago, I didn't know much except how to install a soundcard and add more memory. Even THINKING about touching the motherboard was an absolute no-no. I'd installed OSess plenty of times at work and had done some "help desk" (i.e. "my printer won't print..whine..."

type stuff, but no "nuts and bolts" pc work.
I finally got the um...globes

to put together my own system. With the help of this forum and the great folks that peruse it, I got my system up and running. Guess what? I did it for about 2/3 the cost of a comparable "name brand" system. I know every single nut, bolt, and cable that went into it's construction, because I was it's Daddy. (This is getting sickening, I know, but I'll continue!!!) I could take it apart with my eyes closed. Literally. I think that's a cool thing, man.
There's just no comparison. I'd encourage anyone with basic mechanical skills and a willingness to learn to build their own computer. Once you do it, it's like "holy crap, why was I so scared?" Yes, I had a few hurdles. When I first built this monster, it worked for two days, then it wouldn't boot at all. I panicked. Being that IT WAS WORKING BEFORE, I reasoned that I had a bad mobo. Not the case. I didn't seat the memory properly to begin with. Possibly the bumping from me carrying the case from my "computer station" (corner of my bedroom

) to the kitchen table to install the PCI cards one by one loosened it, that's why the system wouldn't even boot to BIOS. If this happened again, the DIMMs would be the first thing I check. But back then , I lost my freakin' mind. Others calmly suggested things to check. Guess what? They were right. Know how they knew? Because the same darn thing happened to them, at some point in time. They were nice enough to help out a newbie. I try and do the same thing now.
This has been a Mr. Mike Testimonial. Tune in same time next week for more Pointless Ravings.