I just switched from Blackberry data with Cingular to T-Mobile, having had a T-Mobile voice plan for the last four years. T-Mobile is by far cheaper. The only problem I'm having is they don't support MSN email, the work around I'm using is have msn email forwarded to my device via desktop outlook then, if I need to respond, log into mobile.msn.com. T-Mobile coverage isn't nearly as good as Verizon or Cingular but it works without a hitch for me in the NYC metro area.
I don't believe any of the services offer a BES but you can find third party solutions. If you don't need a secure access to MS Exchange with BES, there are other less expensive solutions than the BB.