WM5 & SP2
With Microsoft introducing the Push service in Exchange SP2 (as a FREE upgrade), and a built in utility in Mobile 5.0 to take advantage of this, along with the fact that most corporate servers are running exchange in the first place, can you tell me that you're going to fork out an extra couple grand in costs for a) the Enterprise server, b) retraining, and c) support, for the Blackberry Solution?
Add to that the fact that the expandability and robustness of the WM platform (in comparison to the Blackberry OS) and I honestly can't see the Blackberry being a viable corporate utility any more.
On the other hand though:
For the independent businessperson who doesn't have a corporate infrastructure behind them, I would definitely go for Blackberry. The Visto solutions (Vodafone's Business Email being a prime example) are shaky as the proverbial expletive. From a consumer point of view, if you're a small business/sole trader who needs a mobile email solution, Blackberry is as simple as it gets. It does exactly what it says on the tin, as it were. With the Visto solutions, there are so many administrative bodges on the network side that can be made, along with the wrong configuration file being sent to the handset, having to set up an Internet (vs. WAP) connection on the phone, provisioning the account for the 5 or 6 different SOCs that are needed...etc, etc, etc.
A Blackberry you pull out of the box, sign up on the website and (handset hardware allowing) you're away.
In summary: For a corporate infrastructure that has Exchange in place, SP2 and WM5 are by far the most superior solutions, but if it's a small business situation with little or no IT infrastructure, Blackberry offers the easiest (if not necessarily the best) setup for mobile email.