I'm sorry but I fail to see the downside. It's cheaper, you learn how shit works and .... i dunno I just feel great after it's all done and working, casue I "made" it with my own hands.
Prebuilt it's not failproof.
Good Luck either way.
P.S.
How many ppl did you see that built a PC and than wished they'd bought a prebuilt? now viceversa.......thank you