I've read a number of posts on this topic and I'm still unclear. I have a Blackberry 8330. I pay for it myself; my employer does not pay for my communications. However, I do access my work Exchange server (among other things). Bo I need to pay $44.99 for the BES plan, or am I OK with the $29.99 BIS plan?
If they have the Blackberry Enterprise Server loaded at your workplace, then you can use the BES, which is $44.99. The main difference, is you can wirelessly sync your Outlook contacts and calendar with your Blackberry.
If your work email has a web interface you coulc very possibly get by with the less expensive BIS service. For example, my workplace uses GroupWise and I am able to access it with a BIS service. If you can access your work email with MS Outlook, you should be able to get by with BIS service also.