I've been using mine regularly for 3 years now. I'm on my second iPAQ, ran over the first one with my truck
I envy you the Quickbooks interface. Should be ideal. I'm committed to Quicken Home & Business. I wonder if they'll expand the interface my level?
I do a lot of interaction via email, so, automatically having the last 30 days worth in the PPC has been handy.
What surprised me is how I've taken to using the Calendar, Task List and Contacts. I, early on, committed to MS Office, including the full version of Outlook. Until I started using the PPC, I only used Outlook for email and as a record keeper for my contacts, and still had a paper planner. No More!
Phone calls and meetings and the ongoing Task notes (labor & material) all go in, were ever I am. Using the "Reminder" feature in the Calendar has dramatically improved my relationship to upcoming things. . .I am much less prone to double schedule or over commit, and, because I have a automatic heads up a couple days in advance, I am less stressed about the "surprise" of what I've got to do.
The ability of the device to take dictation or record conversations should not be overlooked. A very small use of RAM (when the format is set to 8-bit, Mono) will yield great result.