Whisperwind isn't as crowded as most servers, but it can get packed on weekends.
Whisper is one of the more crowded servers out there, since it was one of the first. Ironforge and the AH in particular on the Alliance side are incredibly laggy and busy. A lot of the questing areas are very busy as well. Whisperwind doesn't have the severe problems of the top servers though, but pretty close I'd guess. This weekend was great, multiple reboots for example, with the Auction House not working all of Sunday morning, and parts of Saturday.
With all that said, there are two great guilds with people from these forums and OO there. Almost Heroes on the Alliance makes it worth playing there, at least for me. I really enjoy the guild. I'm sure the Wanderers is the same way. Great groups of people that make up for the few problems.
I'm also not convinced that even some of the less crowded servers don't have problems as well.