VPN in to Exchange Server with iPhone 4S?

My wife recently got a 4S and she would like to be able to connect to the Microsoft Exchange Server at her office and work with her business emails and other files.

I notice there are many VPN clients in the app store. Any comments on which one(s) might be the most troublefree and easy to use? And what about "hooking" up to the Exchange Server at her office? Suggestions to make the process less frustrating?

Thanks!