One thing to update this thread with:

BES functionality doesn't require a multi-thousand dollar BES and a $45 BES plan any more. I have the BES Express installed on our SBS2003 exchange server (free) and it works with my $30 BIS plan. You get all of the mail, calendar, task, notepad, and contact syncing that you get with a full BES - but not as many policy management options.

On the other hand, if you use Google for your mail host, BIS has improved quite a bit. The mail delivery is nearly instant, and getting contacts & calendar synced is not a problem. No need for the separate gmail app. I have my corporate exchange email and my personal gmail both set up on my blackberry. In terms of functionality they're very very close. I do use the google sync app to bring in multiple calendars to my BB, but if you're just using a single calendar I think (am not certain) BIS will take care of that.