I have built my own for more years then I care to remember. You can get a great unit for a good price. My last 4 computers, I have purchased HP units and have nothing but good things to say about them. The prices have dropped on full systems including the FP monitors. For me it doesn't pay to do it myself. By the time you add in the software packages they include as well as the convenience of just taking it out of the box and plugging it in, the choice for me is clear.
Another thing to look at is computability issues with different parts that a DYI may be using. Not all parts work together well. I can't afford the time and inconvenience this may cause.
I did get a "Cow Box" computer that failed out of the box and offered POOR customer support. It was a business computer and I would have been without it for 2-4 weeks while it was being repaired. I promptly CHUCKED it into the back of my pickup (Can you say Pent up aggression?) and returned it to the store, replaced it with an HP and never looked back.