IO540, I've not evaluated how much of the add-on software I've used works successfully, but I think you paint too bleak a picture. I mention it because I am reminded of the time I was thinking of buying a TiVo device. I read some forums on the Internet and I had taken notice of the general consensus I'd never have bought one (pre Sky+ days) and yet I did and it has been fantastic.
Your success ratio might go up considerably if you buy software, rather than try to use the various 'free' or very cheap programs available. As opposed to the general market in open source software (which can be VERY reliable - PPRuNe is testament to this), you don't generally get big communities of developers working on a PDA application. Your moving map example is one that I feel supports this notion. There are lots of very happy AnywhereMap users (just not in Europe) and I've personally used Memory-Map on 3 different ipaqs and it has crashed only two or three times (about as many as my Garmin 196). I've tried various pieces of software like Pocket FMS and though that particular one has improved a lot with time, I've had much less success and have given up on them.
Back to the original question - I think a lot of the problem with a combined phone and PDA is that for me to like a phone it needs to work well one handed, ideally with physical numeric keys, so that I can dial and text without having to give the device 100% of my attention (hey, someone has to capture that localiser and our AP doesn't work). For a PDA I personally like a thumb keyboard (in particular because I have to use a PDA sometimes to connect to servers, so predictive text is no good and an on-screen keyboard takes too much space) but these tend to be very small, so don't meet the requirements for a phone. I want a phone to be as small as possible because I have to have it with me all the time, but I want the PDA's screen to be big - hence no one device is likely to please me.
I tried a SE P910i because it was a decent compromise, with the screen as big as the device, a numeric keypad and a thumb keyboard, but the software is dreadful. You had to use the stylus to do things that so easily could have been done with the keyboard. The fonts were ridiculous for a device with a screen like that. I think the problem is that the OS is a generic one, not designed specifically for the P910i's particularly hardware (a touchscreen, a thumb keyboard and a four-way wheel).
The user interface on the popular Blackberry (the 7230 - I've not tried the latest more phone-like one) is fantastic - there are lots of key short cuts and the wheel works well (it's better utilised than on the P910i). It's not the best phone and the original ones didn't have Bluetooth. Even the new ones that do cannot be used as a modem; the BT is just for a headset. The other problem with Blackberry is the very limited range of add-on software.
The Treo 600 and 650 are quite good too, if you like Palm OS. The 600 lacked bluetooth which was a pain. But my 600 broke really quickly and the screen resolution was very low. Also I don't like the antenna poking out the top - don't know why they need to do this when so many others don't.
I could go on and on! My livelihood depends on the functionality provided by these devices, so I keep trying them to find the best to suit me. My personal favourite at the moment is a Motorola V3 phone (because it is small, has a loud ring and feels like it is good quality in your hands) and an ipaq 4350 (because it is has a good built in thumb keyboard, is reliable and runs all the software I need).