We've been 3 times at that time of year. We love it, but you want to be prepared for anything weather-wise. I can remember frost warnings on some evenings (they seem rather shocked about it on the TV weather, so it must be pretty unusual). And in the mornings and evenings, my DD would need mittens, coat, etc. Sometimes a jacket all day. Some days it's been 80 degrees. There have been a few rainy days, but not the daily afternoon downpours that I hear can occur in summer. I can think of only one day where rain was a problem. It started misting which wasn't bad, but then it poured...and poured. It got pretty uncomfortable, because it was very chilly.
We went back to the room, changed into dry clothes, and by afternoon, the rain had stopped.
It's our favorite time to go. We have never done any swimming. The pools are heated nicely, but I always worry about the walk back to the room in the chilly air.