There aren't actually many fair/accurate comparisons around since most companies have either a shrink-wrapped product or a webapp. Rarely do you see both, and when there are I agree with jacquesm that the experience is almost always sub-par on the webapp [in comparison to the desktop app].

You can't really compare one company's webapp to another's non-webapp. i.e. you may feel GMail is better than Outlook, but since Google doesn't make a non-webapp version of GMail we don't know if that would be better than both Outlook and gmail.com. Microsoft does make a webapp version of Outlook, but I don't think many people find that better than "real" Outlook.