All I will say is that nothing changes the characteristics of a bike very quickly as great wheels. I currently have eight wheelsets, and I can tell you each of those wheelset have different effect on my rides.
Yeah, a great wheelset can make for great fun on a road bicycle. I don't have similar experience on a mountain bike, as I only have the wheelset that came stock with my mountain bike.
Great road wheelsets that are stiff, aero and have great rolling hubs are the cat's meow!