You should replace your wheels when you taco them. Or, alternatively, if you start popping spokes--the general rule is 3-and-out.

There are many good manufacturers out there, and it really depends on the riding you do. If you're just commuting or riding for exercise, you'll probably want to stick to something with more spokes for structural integrity. My fixed-gear has a set of Mavic Open Sports, and they take quite a lot of punishment. But if you're riding club rides with racers, you're going to want to shed some spokes and potential switch to carbon wheels.