The Cons as far as i am concerned are
1. No warranty
2. No one to turn to if it all goes wrong
Everything else is a PRO,
feeling of achievement in building your own system.
getting exactly what you want without all the "free extras".
the ability to upgrade without fear, cos when you have built your own you have done it all anyway.
only installing the software you want. no extras like "Freeserve" etc.
+ you also know Exactly whats in your system. no suprises when you format and cant find the drivers cos the shop didnt supply them.
Good Luck building your own computer, Just dont rush it mate, nice and easy does it.