Building a PC is a great experience, I highly recommend it. A little frustrating (especially if you do it alone with no help like me), but very satisfying and rewarding once completed. Just knowing that you built it yourself makes you love it that much more, and makes it that much sweeter when you power her on.
Upgrading your PC is very easy. Also if anything ever goes wrong...instead of having no option aside from taking it to a repair shop/dealing with warranties, you can actually do some investigating, and maybe even crack her open and do some troubleshooting/fixing yourself!