I've gone both ways -- build up a frame with the components I want, and bought complete bikes. Hands down, buying a complete bike is the cheaper option. You just can't beat the price that manufacturers can get their components for.

That said, I like getting to pick and choose, oh - and if you find a complete bike, but you'll need to swap out a bunch of things to get it to fit, the scale swings back the other way -- it would be easier to start from scratch.