In terms of efficiency and effectiveness of connection utilization, BB is top notch.

They have many optimizations to lower the bandwidth usage (data compression, optimized email workflow, attachment optimization, image preview system, batch processing unimportant system messages, etc) and they provide encryption between your BB and the BIS/BES server too..

Though I don't use the same BB provider as my other devices, it's also the most stable (in terms of maintaining network connection and all services running on it) compared to my other devices (iPhone 3GS, N900).