I have BIS, that's the problem... BIS doesn't offer over the air syncing of calendar, contacts, etc.. the way BES does. When I make a change in Outlook or on my device, I'd like the other to recognize it immediately.
You can get OTA syncing of the Calendar and Outlook (albeit in a roundabout way) by using the Google Sync for Mobile on your BB (to sync to a Google calendar) then using the Google Calendar Sync on your desktop to sync that Google calendar with your Outlook calendar. You can set it up to work bidirectionally or one way (whichever way you prefer). Works with Outlook 2003 and 2007.
So with Google Calendar and Gmail, you get push email and wireless calendar sync, two of the big advantages of BES.
Though I agree that a "personal" BES type of app would be ideal. But this is free and available now.