We stopped supporting BES (Blackberry Enterprise Server) and Exchange a long time ago due to its cost, stability, and complexity. We know lots of folks are fans of Blackberry’s so we looked all over for an alternative.
AstraSync is what we found and have recommended to our clients for years. It works well and our Blackberry users seem to be happy with it.
Hopefully one day Blackberry will just license ActiveSync like Apple’s iPhone did, but for now, AstraSync works great and is cost-effective considering the alternative.
So if you need your Blackberry to talk to Exchange, get AstraSync and move on to more interesting things 
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Easy Blackberry Sync with Exchange…
Post from: Ramblings from The Montopolis Group Easy Blackberry Sync with Exchange…
Check this out… BES Express http://na.blackberry.com/eng/services/business/server/express/.
No licensing costs, totally free.
BES enterprise supposedly supports 2k users, but beyond 1200-1500 users causes delays. Not sure how that translates on BES Express. I’d imagine you would want to isolate the database if you go beyond 75.