This is a very basic reason for choosing a class library project for websites in Whidbey: Compiling is faster. Yes, the IDE has the option to “Build website”, but this “validating” step takes forever, and in my class library, I can build the same stuff in a fraction of the time, and it actually goes into a single binary instead of hundreds.
Another hiccup is that the auto-gen partial class mechanism doesn’t keep up all the time, and I’m left with build errors not because my code is wrong but because the IDE is behind on updating the other half of my code behind. Stick with the CodeBehind model of v1.1, and I keep my protected control declarations _and_ I don’t depend on the IDE to generate half my code. No false build errors.