I have had experiences with companies using Exchange and Gmail, here are my comments:
Exchange (especially 2007) has many many many features, some of which you will never use. It can even manage voice mail integration.
If you have >100 users, rely on email for your business or have any sort of complicated requirements such as shared folders, shared mailboxes e.t.c. I think there is nothing better than Exchange. I have a Windows powered phone and have my contact list, todo list and notes synchronise to my phone and outlook... I have set up complicated policies for users such as some can only send to some addresses, everything archived in a place, some users can share others mailboxes, some can send on behalf of other users (assistants e.t.c.)
Additional servers are provisioned easily through the wizards, Basically, it just works, and works very well. (also, Nokia, RIM, Apple all work against it well)
Google on the other hand, I think for small companies or places that do not have a lot of emails works just as well. At the end of the day, you can send and receive just fine. The sync features have gotten a lot better. Also for smaller companies, it is probably cheaper for Google to host it than the electricity bill of having a server on 24x7x365.
For larger companies, whilst I know some do use Google, I really would not recommend it, it is doable, but it simply lacks the real business features that some of my clients (in 500+employee companies) rely on for their business.