Whenever I'm evaluating a client application that interacts with a web-based service via its API, my acid test is the answer to this question: "Could I fully utilize this service using only this application?". Satchel gets you most of the way there. With existing Backpack pages, you can view all of your notes and lists, edit the pages tags and even view attached images and files. You can also create new pages, but the API doesn't allow for creating anything other than Note and List elements within your new page (though, you can email files, etc. directly to it once it's created). The good news is, Satchel also has a "send an email to this page" button, so you can add content to your new page relatively easily. The Satchel crew also really spent some time on the list interface, making it easy to add, remove and reorder list items in a very iPhone-ish way.
Personally, I'm not a big user of the Backpack calendar (Google Calendar, if you're wondering), but Satchel makes interacting with your Backpack calendar a snap (at least, my relatively limited testing would suggest as much). The bad news is that you're not able to edit events on the calendar (or delete them) because this functionality isn't exposed by the Backpack API. I'm told that if the API does support this at some point, then so will Satchel. Otherwise, it looks and feels very similar to the stock Calendar app that ships with the iPhone, so it'll probably take new users all of three seconds to make sense of the interface. It also allows you to drill into specific calendars (if importing additional calendars via iCal feed is your bag). Reminders also work as advertised (which is huge for me, as I make pretty heavy use of this particular feature.
While you may be a little hesitant to drop the $9.99US on this one, I can say that it's worth every penny for the true Backpack fanatic. It's gone a long way to removing the barrier for those looking to embrace Backpack as a service, but feeling a little hamstrung by the lack of a decent mobile interface. If you love Backpack, you'll love Satchel [App Store].