First, I sell commercial mail servers (no, not Exchange), so I have some bias. I'm not going to plug the product I sell (though it's easy enough to track it down through my profile if you really wanted to).
If your small business is 10 employees or less, I think Gmail is an ideal choice. I use it myself, and although I have some gripes about this and that, overall it beats everything else out there.
Above 10 employees, things change and it becomes time to consider an in-house server. You are going to want and need the calendering and collaboration apps that are not included in Gmail's free version, and you also need the flexibility, control and security that can only come from an in-house server.
There's also the matter of cost. At $50 per user per year for the full features set, Gmail through Google Apps is quite expensive. At 50 users, even including the cost of hardware and maintenance, an in-house server is half the cost or less of using Google.
I have links that talk about these things in more detail, but I am new to this community and don't know what the protocols are here so I am not including them.
What did you think of this review?
Use Trust Points to see how much you can rely on this review.