As you said, nothing is free.
Mobile broadband is - in my opinion - a misleading name. Broadband is by nature anything above a 512kbps service and should offer AT LEAST 1Mbit. Now, you may get a fast service though clever compression and other whizzy bits, but in reality outside major cities and airports you are going to struggle to get service at reasonable speeds. Vodafone offers a high speed service in airports and major cities and high population areas, but beware - if you fall outside this, you will not get what you expect and there's little you can do about it.
T-Mobile also offers web-'n-walk which is OK but quite frankly it is down to what your expectations are.
If you want to have true mobile broadband and you live outside a major city, then forget it. You'll possibly get low speed or medium, but you only have to look at Vodafone's coverage map for mobile broadband to get a good view of service levels. For the word 'variable' read 'almost non-existent' as my business tested this in alleged 'variable' areas and we got virtually nothing.
Don't quite understand your reference to MS Exchange - has nothing to do with mobile broadband!
SHARK!
For every positive action, there's an equal and opposite government plan.