We were at WDW when the Dinorama area of AK was previewed for passholders. We also thought - WOW, there shouldn't be a big crowd - let's go!
Boy, were we surprised. I have NEVER seen so many people in one park in my life (and we make very frequent trips). It was so crowded that after we got our free ice cream and pop and took a quick look around, we just had to leave. There was no way we were going to even close to the rides in less than an hour. We went back a couple of days later and it was much more reasonable.
Now, they held a special event just for this and it was just one evening only, so maybe that contributed to the crowds. But, if I was going in April or May, I would expect it to be much more crowded than usual. That way if it isn't bad, you are pleasantly surprised.
(Now, let's add that I WISH we were going then - but we have to wait for June and fight the summer crowds!)