I keep mine fed from BES server at work.
I keep all of my recipes in the memo pad.
I keep my life organized in the calendar.
I keep all of my contacts (cell numbers, email, SMS, mailing addresses, etc) in the address book which is replicated back to the BES server at work. Then from there it is replicated to all of my other PCs. So once I enter contact information at one place (at the PC or on the BB) i know it gets copied everywhere else. So I only have to maintain one address book (and never worry about losing it).
I use google maps for driving directions. At times I wish google maps was integrated with the BB address book. That way I could select maps targets using existing contacts, or save a map search to an address book record. Actually, it would be nice if when browsing a current address book entry, if I would ideally just scroll in the track wheel and select "route to this address using google maps and current GPS location." That would be the bomb.