There are many providers to choose from when it comes to Hosted Microsoft Exchange. Prices vary, but most of the features are pretty standard today. Just make sure they have all the additional features you need, such as ActiveSync, Blackberry Wireless Sync, Good Mobile Messaging, SharePoint etc..
Anytime you select an outside contractor, I always recommend tossing aside the least expensive. They generally have the lowest quality and are the least reliable. Remember - You get what you pay for!
For most people I would say that uptime, and customer service are the two main factors that come to mind when selecting a provider. Be sure to contact a few providers before making any selections.
Send an email to the sales and support email addresses. See how long it takes for a response, and check the quality of the response.
Visit their website, and see if they offer real-time online chat. If so, Start a chat and see how long they keep you waiting. Ask questions that you think they should know. Make sure they meet your expectations.
Place a phone call to the sales and support office.
Do they answer the phone? - Leave you on hold?
Finally - don't be afraid to test the services of these providers before making the final plunge. Many of them have a 30 day money back guarantee and no contracts. So testing a few should be fairly easy. Be sure to verify they have a money back guarantee.
This might help you get a better idea of the true level of support and service they can provide.Good Luck!