I will Google for an answer first. However, sometimes making the query return the results your looking for can be difficult. Sometimes the results aren't quite what you need. Or the answers are out of date and have been superceded by updates so they don't answer your current needs.
I also search on SO for answers. Usually it's hit or miss. I have the same problems I do with Google: making the query return something useful. I don't know how many times I've asked a question and had someone tell me it's redundant.
However, that's the beauty of SO. You get responses quickly and even redundant questions can have answers. This is good because there are new people here all the time who many not have seen the older posts.
In general, I post questions to SO when I have worked on the problem for a day or two and either can't figure it out on my own or can't find an answer on Google. I will also resort to SO when certain web sites are blocked by the command's firewall (I work for the military and many blogs, forums, etc. are blocked).
Finally, SO is a good, central repository of knowledge. There have been many times I have found an answer here or gained some knowledge that I didn't even know I wanted. Due to the variety of questions being asked, I never know when I may find something that will prove useful later on.
As a "payment" to the community, I provide answers when I can and I try to ask questions that others may find useful.