I haven't been there, but heard and read plenty about it. First of all the place is huge, and it probably offers the best terrain in north America. It has it all glaciers, bowls, chutes, steeps, glades, groomers, cliffs ect. People claim that you need at least 4 days to really understand what the two mountains are like. They also have amazing terrain parks. For the 08/09 season they are opening the peak-to-peak gondola this is going to make traveling to from mountain to mountain doable in one day, it might also help the crowd factor.
The 3 problems I hear of are rain, $$$ and crowds. A acquaintances of mine was there last season in march and he experienced rain 4 days out of the 6, people tend to complain that it tends to rain a lot in the lower part of the mountain.
Crowds can be avoided if you know where to go, not that I know where to go, but thats what I heard.
And the $$$ is a big factor the place is fucking expensive, you would have to shop around to get some acceptable deal on lodging and stuff.
Whistler is a resort you have to experience at least once, I plan on going maybe this coming season or the next one.