Welcome to the forum, and thanks for your first post.
BIS has consumer routes to it , and all most consumers are worried about is the sync of their inbox, the ability to have internet, some IM clients and a good device.
As you're hosing our own domain, I'd expect that you'd be wanting more from your device than just that.
As you host your own domain (and I'm guessing exchange/domino sever??) thne you may be interested in BPS (BlackBerry Professional Server) BlackBerry - BlackBerry Professional Software
which is pretty much a fully fledged BES but limited in (very few) areas, and limits you to 30 users.
BPS is free from RIM (BlackBerry) and comes with 1 free CAL for your device. This will give you so much more that a Windows mobile device could ever give you, all be it most at the back end.
You've got to remember that BlackBerry is born from corporate roots, and has enterprise functionality at it's heart.
There is so so much that a BlackBerry can offer you over a windows mobile device and there are plenty of threads that can give you the pros and cons if you do a quick search on this site.