I’m a big fan of 37 Signals products, and do extensively use BackPack and Basecamp products.
I used to use Satchel for iPhone (the only available iPone application for Backpack). Except for a few bugs and usability issues, I became addicted to it. The version for iPad is also a great job form the guys of Stand Alone !
Quick tip : if you are stuck in the Calendar section of the tool, try to tilt your iPad 90 degrees, it could be because you are in a portrait format instead of landscape and there is a bug, the “page” button is hidden in the portrait display :-)