The first thing I had to do was stop thinking it was a laptop. It just isn't the same.
Now it doesn't help that the current OS is a throwback to the dark ages. Some have compared it to Mac classic, but even the Mac could keep multiple apps running at the same time. It's really like DOS: one app running at a time, each with a slightly different interface (some supporting the newer navigation buttons better than others), each being loaded into memory and then pushed back out again when done.
Now a few apps have hacked a sort of background run mode (kind of like the old borland popup interrupt utility that could bring up a notepad, calculator, and calendar; while in another program), so you can play music in the background or stay connected to an Instant Messaging service while doing other things, but by in large, most apps shut down when you switch to something else.
One of the most annoying examples of this is the browser. You have to sit there and watch it load, and wait for it to finish completely to even have a chance for it to keep its page around when you switch out to another app and the back. Even then it's likely to have to reload from scratch, if it doesn't forget about the page and the link history all together.
So after unlearning all my old habits, I set out to learn some tasks that would be effective on the Treo.
Trying to solve old problems with new technology is rarely productive. Thats one thing that makes learning new tools difficult. Without someone guiding you through new problems while you get your feet wet, you end up projecting each feature onto old problems and comparing it with other tools you're much better acquainted with.
This is what has made it so hard to learn a new programming language. I already know several, and when I start learning features in Lua or OCaml, I compare them against perl or C++ and for old problems I know, these tools are much better. It doesn't help that one of the favorite things for functional programming people is language parsing & AI, two topics I have very little interest in.
So one big advantage the Treo has over my laptop, is availability. The treo fits handily in my pocket, and I don't mind carrying it around, unlike my goliath class laptop. Its form factor is also more convenient in tight spaces, like in the cramped quarters they refer to as coach class on "modern" airlines. Battery life is another advantage, with the Treo lasting all day on one charge, even while playing music and using the internet. (The comparison is a little one-sided at this point, just because my laptop battery is close to dead and lasts maybe 3 or 4 minutes.)
So given that browsing is brain-damaged, the first killer app is email. With the ability to pull new mail quickly and the small fonts for viewing, working with email is great. My only request (besides wanting an update to versamail that doesn't reset the whole unit constantly and corrupt all the memory setting) would be a shortcut to pull new email for all my boxes (I currently have four different accounts setup).
Reading is also pretty good, though the Adobe eBook reader won't switch to smaller fonts. Loading reading material takes a little work as well, but I've managed to get a number of documents loaded.
The most surprising killer app is writing. Though the keyboard is not as convenient as a full size one, I can still type out a couple dozen words a minute, with enough time to spare to think about what I'm writing. While I don't pound out thousands of words in a session, I can eventually get at least a few ideas written down.
I'm still working on this thing, figuring out what's what, but I'm willing to declare this gadget useful. At least until the next big thing comes along.