Hi there,
I've upgraded wheelsets twice and have been happy with the improved performance each time. Although, I'd have to say, the OEM wheelsets also performed adequately and reliably. My upgrades have been more about want than need. I replaced an entry level Easton set on my Giant OCR3 with a wheelset from Performance Cycle (Forte Titan brand). There were rock solid and carried my 185lb frame over 20,000 KM's. They only cost $165 for the pair (a blowout sale, regular over $300) and the bladed spokes and semi-aero rims looked great. I've sold the Giant now and upgraded to a Colnago. Just a few weeks ago I upgraded the Shimano R500 wheelset to Mavic Ksyrium SL's. Wow, if smooth has a name, this is it. They cost a lot (as much as the entire Giant cost me 6 years ago) but it's absolutely the single greatest change in performance that I've noticed on any of my equipment upgrades over the years. I'd say wheelsets are a good bang for your upgrade dollar.