Dymag and Marchesini, and to a lesser extent Brembo, as well.
Unless you have a seriously modded bike or you're going racing I see little point- I'm sure the results are as massive as we're told but the cost of a set of good quality lightweight wheels is knocking on half what I paid for the whole bike
It seems you'd need to make upgrades to the suspension to get the best out of them as well. Also, they're less durable, and vulnerable to crash damage.
Certainly makes more sense than throwing £1500 at engine work, mind... I'd like a set myself but I can't justify the cost.