I don't really get the question, do you mean, custom or pre-made?
With a custom build, you know exactly what's going inside the computer. If you ever want to upgrade or want something special when you build it(that may not be offered by computer companies) doing it yourself is the way to go. However, you don't always know every part will work(DOA's and such). Plus, at times, there can be minor incompatibilities with computer parts. Typically RAM is the culprit.