I'm in the camp that new wheels can make a big difference, but it depends on what you're coming from compared to what you're getting.

My first road bike came with a set of boat anchors. I didn't know they were boat anchors until I got a new set of wheels--Vuelta Corsa Lites. The first ride with the Vueltas was a revelation. Over the same courses, my average speed substantially increased.

When I upgraded my bike frame, I went to a nice set of aero carbon wheels. They weighed a little less than the Vueltas and were better in terms of rolling resistance. I was hoping for a big performance boost, but they really didn't make that much of a difference. YMMV.