I've been using Backpack since the launch earlier this month.  I was initially attracted to Backpack because of the technology involved (Rails, AJAX), but I just became a paying customer because of the pure utility and simplicity of the service.

We all have todo lists - once the list gets beyond 5-7 items, it is usually a good idea to write them all down.  I've used the task features in Outlook extensively; I even used Taskline for a while, a cool Outlook plugin that maps tasks into calendar entries.  My main issue with using Outlook tasks is that they aren't as portable as I'd like them to be, even with synchronization to my BlackBerry.  Currently this synchronization only happens when docked, not wirelessly.

I also have a long list of items I track across a range of projects, some work-related and some personal.  Many of my personal projects involve collaboration with others - often via emailed lists of assigned tasks.  So allow me to enumerate some of the very cool features of Backpack that I'm currently using:

• Create pages for each of my projects.  On each page I can track a todo-list, notes, and (because I upgraded to the fee-based service) file/image uploads.  The data entry via AJAX is extremely slick.  There are a few kinks they still need to work out (especially around focus for editing list items).
• Share pages with the public or specific individuals.  For non-sensitive info, I just make a page public and share with my collaborators.  This allows them to view the page (read-only) without having to create a Backpack account.  Here's an example page I've made public:

• Access my Backpack from my mobile Blackberry browser via the mobile interface.  No AJAX support there of course, but the interface is clean enough and is well-designed for my most common use case: checking off task items.
• The killer feature, in my book: emailing items to my backpack. Each page has a unique, system-generated email address that can be used as a mailbox for adding content to a page.  I added these addresses to my contact list ("Backpack Personal", "Backpack Chess", etc.) and when I think of a new todo item, I email the appropriate Backpack with "Todo: Take care of FooBar" and seconds later it is added to my list.
• Use the reminder service to generate SMS reminder messages at specific times.  There are some great shortcuts for reminders as well.

My recommendation: create a free test account and try it for a few weeks.  If you don't spend much time at a computer, you may not find it as useful as I do.  But if you do, Backpack is a great way to keep your personal life in order.