Interesting topic... you gotta brake it down into what you wanna do...
Ive always built my own systems, but don't know if i will build my next rig myself..
Reasons to self build:
1, Build to your exact own spec
2, You know what you are getting
3, Its fun (sometimes).
I personally think the money saving is not the reason to self build anymore,because if you shop around you don't save that much money unless you want some wacky spec
Reasons to by prebuilt
1, Less hassle
2, If it dont work you dont have to worry its the suppliers problem
3, Faster.
I have only given 3 points to each so to leave the amounts equal. and this is a very generic look at this.. there are always differing reasons depending on your requirments so some may not apply or more may...
well thers my two pence worth