It's pretty much the same as everything else in life: experience is king.

One must know their limits and when it's ok to pay for professional help.

It is not evil to have a good LBS you like to do work, and should never be discouraged, as what some here think.

I know folks that believe they should DIY everything in their life. They screw everything up, everything they have is shoddy and crap, and they seem proud of it.

I know folks that spend more time on DIY fixing their bikes that riding them. I'm simply happy it was 0% of my time on their broken crap.

Having said all that, I DIY because I'm a bike wrench that has worked in LBSes, and I resell bikes at a business sustainable profit.