Hi. I get asked this alot. I build my own, and sometimes for my friends. Most most often I suggest that they buy a name-brand PC (usually Dell or HP/Compaq).
For others, it boils down to how much they want me to continue helping/supporting them. If they are next door, I'll build them one. If they are more than one area code away, I tell them to " get a Dell, dude."
...and my reason is always the same: Windows gradually gets mixed up, slows down, and becomes hamstrung with registry issues and other problems. At that point we all ask ourselves "why is this computer slower than when I bought it, since I'm running the same programs".
That moment happens somewhere between 6 and 12 months, depending on how much software you install and how much spyware gets into your system. That is often the point at which it is better to format the hard drive and do a "fresh" windows install.
In school, we learned that the three "R"s are Reading, 'riting, and 'rithmatic... In computer science, they are Reboot, Reset, and Re-install.
So, I build them myself to make sure I can get back to "factory fresh" as fast and reliably as possible. That's why I recommend Dell & HP for those that are too far away for me to help with that. Both Dell and HP have a history of putting good restore mechanisms in their systems so users can have a "do over" periodically. It puts the user in control and helps to mitigate the inevitable impoding that Windows continues to experience (although it is much improved with XP - finally).
The other benefit to me is that I can mix and match parts to make better use of the inevitable pile of parts that I accumulate as I upgrade or change my PCs. As a bonus, I periodically find I have enough parts left over to build a decent PC for someone heading off to college, or for use in the church office, etc.
As far as cost, I don't think it costs more or less... I try to keep it about the same. My rule of thumb is:
"Can I build to last-year's Alienware desktop level for half of what I can buy the current state-of-the-art system." I use Alienware because they tend to stay at the high-end of components and adopt them early - when they are most expensive. (You can use Voo-Doo, Falcon NW, or whomever you consider to be the best.)
So, if the current top-of-the-line rig would be $3,000 from Dell/HP/Alienware, I should be able to get close to their previous model for less than $1500, and I should be able to build a budget PC (2 models behind) from spare parts for about $500-$600.
Example: Current hi-end is a P4-3.6GHz, I would consider last year's to be P4-3.0GHz, and an entry level to be P4 (or Celeron)-2.66 and below. Sure enough, you will find the breakdown for new systems to be $3,000 - $1,500 - $600.
This generally works out, gives me maximum flexibility/control, and efficient use/re-use of the various parts I buy and/or replace over each PC's life-span.
All that sounds complicated, but it allows me to be concise and helpful when people ask me for advice. I can ask one or two questions about where and how they will use the PC, and I can recommend a course of action and target price with complete confidence that they will be happy with the results. They don't ask because they want to hear "that depends", they ask because they want guidance and simplicity and reasonable expectationsfor price and capability.
I hope this helps someone out there!
As an aside, the big pain for me will be when I have to finally adopt PCI-Express, as it represents the biggest cost and least compatibility with the MoBo's and video cards I have today... I better get some of that on my Christmas list for 2005, and I better start asking now...