My 2 cents...
BES is by far the best enterprise messaging platform out there. RIM's network is extremely reliable and as far as I know, hasn't gone down except for a few extremely well publicized incidents a few years ago. The centralized management features are absolutely fantastic.
That being said, if they have a Storm, that means Verizon. Go to Verizon's BB link and set up a BB account for them. You can then sync the email via OWA. Jason is correct in that it won't sync contacts, calendar, or notes. You need to install the desktop application for that. It does have the side affect of letting them charge the BB during the day.
As a side note, if you have an extra pc lying around, you can get a free single user edition of Blackberry Professional Edition for Exchange. Also, if you do end up getting a bunch of users with BB's, look on eBay for BES licenses. MUCH, MUCH cheaper than RIM.