that's a question everyone discuss last time.
I actually don't prefer desktop apps. I prefer as desktop apps, as a web apps in the own cloud. why:
Security. My data is available only for me and my admins which are under my control. Every company larger than 5 employees has it's own secrets. If Google or government don't need your data in the present time, it doesn't matter they wouldn't need it the future.
Stability. If something fails I can kick ass of my admin. Remember the times when i.e. gmail was down. Even our exchange server has more uptime than gmail, even if we need completely put it in the maintenance, we do it when we want.
Flexibility. I can install any spam filter, any interface templates, any other stuff I want. At 3rd party web app you are bound to the it's functionality and can't change it so flexible.
why google apps are better:
you don't need to buy a software, you can rent it or even use for free with ads and no support.
you don't need to hire admins, that's very good for a small company. we used google apps during a year until we grow enough to hire own admins.
some things are really better i.e. google docs are much better than sharepoint 2007. But google is too lazy last time and there were no any major updates for a years while sharepoint 2010 is coming.
Of course people who have a computer for Facebook, Twitter and IM will enjoy web os at home, but at work - still no. But there's a lot of such people and their home desktops will soon belong to Google :)