Sounds like a good idea, I assume the selling point is that it saves you buying MS Exchange for your company, correct?

Do you know if supports Outlook / Thunderbird use?

hmm, who should want to give Google access to their entire corporate email system?
The security implications (giving all your corporate communications to an outside party who reserves the right to do whatever they want with it including selling it to the highest bidder) outweigh even the massive privacy problems with gmail and Google in general.

There are some very useful applications. Although it may be a poor idea for a company, it is a great idea for a small organization. I'm talking about like an after school club, or a fraternity or something. Or if everyone at Daniweb or some other forum had an email address setup up for them when they became members. It would sure as heck increase the activity for a forum. When you get into forums as large as Dani web there may be issues, but it is ideal for smaller ones.