We recently moved from Exchange to Gmail.
With GMail if you use Outlook, you get no calender, contact list, tasks, or notes without third party sync tools. If you use Public Folders, those are gone. There is no global address list, so every employee has to maintain their own contact list (I forgot how much of a pain this is, as I haven’t had to do this since 1999 when I first worked for a company that had Exchange). If you use folders in Outlook and IMAP, the experience when using web mail is totally different than when using Outlook as Gmail doesn’t use folders. They use tags. Creating rules in Outlook are client side only, so if Outlook is closed the rules don’t run. Google’s filter engine (their version of rules) is lacking when compared to Outlook/Exchange.
The only thing that I have found that hosted Gmail has going for it is the amount of space you get. But with a little work your internal Exchange server can provide the same amount of space.