We use Google Apps for all of our start-up communication/document infrastructure. It works pretty well, and is obviously compatible with most email clients/calendars people prefer to use on their desktop.

One thing to be careful of: Yahoo! Mail has been filing emails sent from our domain (and others I've heard about) directly to the spam folder - and these aren't bulk emails, just person-to-person emails. We had to enter into a lengthy discussion with Yahoo! support about getting our emails marked as ham: and we lost quite a few investment opportunities because of these missing emails.