I've been using an iPaq 3870 and various Bluetooth cellphones for a few years, and it suits me fine. There are drawbacks - the screen isn't big enough for surfing, but it's still a lot bigger than the phone's.
I had the same rollcall of updates and ROM flashes to get the iPaq to do what it should - and even then, it screwed up right royally once and got me into deep trouble. It doesn't display "whole week" events after a bit if you're scrolling round looking for a free day to go to visit someone. So I ended up booked to be in two different countries at once. The cure is simple - don't use the iPaq calendar software; the separate Pocket Informant is brilliant as diary/address book/notebook etc - and good value.
The iPaq has a 512MB SD card, which holds all my data (including who's where in the churchyard) and doesn't lose it if the the battery runs flat.
Drawbacks:
1. Battery life is short - I carry a "battery extender" with four AA batteries in it.
2. Size - even with a small case, it's a lot bigger than a cellphone, although it does do a lot more.
3. Pocket FMS won't run on a 3870 if you use the SD card to store the software. That's some file access bug that nobody's fixed yet.