The Palm and other windows CE devices use the web access feature of Exchange and poll the server - much like a POP3 client. This is all nice and well, but it does suck the life out of your battery if you poll frequently, and your email notifications are delayed until the next poll.
Because the BES (or BPS) is a server application that connects directly to your Exchange server, it intercepts messages as they arrive and push them to the device. As a result, I sometimes get email on my phone BEFORE I get it in my Outlook inbox.
I personally prefer the push technology of the Blackberry product, and the Blackberry Professional Server suites my needs and is less expensive than the Enterprise version. (a one-time cost of $800 for 10 users, or $80 per device). You can also "brick" a users phone through this in the event of loss or theft (or termination). You can configure the server software to allow LAN connections (ie be a vpn server) for your phones. As an example, using an ssh application for my Blackberry Pearl, I'm able to telnet to my server from anywhere. Not that I'm going to do anything productive on a Pearl, but it has been handy for checking status and launching a job on occasion.
There are many other features of the server software, but if all you're doing is setting up sync to Exchange for a handful of users, then yes, the path of least resistance is to go with Palm or the Motorola Q, Q9, etc. I've tried the Q and found the BB devices to be faster and easier to use.
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