I just trued up a pair of Ksyrium Elites today, which brings up what I like about Mavic wheels. The hubs are light and durable, having good cartridge bearings.The rims are also strong, light, and they true up easily once you have the wrench for the proprietary nipple in your hands. Should you break a spoke, though, which occasionally happens, replacement can be a hassle. After over 10k miles, my own Ksyriums (SLs) are rolling great after one truing and adjustment of the front hub, and several cassette body overhauls. More about that below.
I also overhauled the cassette body, which brings up my caveat about these wheels. The Mavic cassette body spins on teflon coated plastic bushing that wears out. It wears out faster if you don't clean it from time to time, like every 1500 (Mavic's recommendation) to 3000 (my actual interval) miles. Ignoring it leads to different consequences, depending on how oblivious you are to the condition of your equipment.
The solution is to learn how to take apart and clean this thing or remember to tell your bike shop mechanic to do it when you take it in for a tune-up.
Overall, I think Ksyriums are pretty dandy wheels. I don't know if I'd go out of my way to buy them, but they don't deserve some of the bad rep they get.