It comes down to budget. When I talk about the prices here I refer to a whole system, not including any tax or shipping, but including the price of apps and OS. If you have components to keep using, then building becomes more of an option at lower price points.
If you want a cheap system, say less than $1000 then buying a dell or equivalent usually comes out the better deal, you usually get crap they had sitting around from discontinued models, and low quality junk some cheap taiwanese manufacturer dumped on them, but you're going to get that building yourself too, at least dell et al. offer a warranty on it.
In the $1000-2000 range it generally comes out about even, the deals the big sellers can offer can sometimes give a price edge, but you generally end up with much higher quality components building yourself in this price range. This is the break-even point for building yourself, since the big corps tend to inflate their systems in this range a little to make some profit that the low-end systems don't. When building yourself here, make some compromises, choose a slightly lower clocked CPU and buy a mobo that can overclock, skip the most recent video card, and buy one generation back i.e. Gf3 or Radeon 8500/8500LE instead of a Gf4, this way you can still afford quality components for the less obvious things, such as major brand RAM.
In my experience, for anything surpassing $2000 building it yourself is the best way to go. The big corps tend to gouge these high-end systems because they know they can get it. They generally still cut some corners here too, using shady PSUs and RAM modules with names on them that you can't pronounce.
Of course YMMV, and picking up the hot deals can build a good computer for significantly less than the big retailers. As everyone above mentioned, building it yourself is a good move looking forward, generally off-the-shelf components have much better upgrade paths available, plus you have complete control over the system, and problems become less of a pain.
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