So far I have done all the maintiance on my cars since I started driving. But I'm still and school and I can't really afford to pay people to work on my cars to often. If I couldn't do my own wrenching I'd have to sell the MB's. Heck I bet I'd have over $30k into the SDL if I took it to a shop for everything.
As I get older I suspect that I will sub out more and more, but I will also drive newer cars that need less.
There are some very good shops out there, and there are a lot of bad ones. Like anything you need to find a good one and pay them well. Remember the mechanics are trying to send there kids to school as well, so if they do a good job pay a fair price for it.