1) Their screens are not powerful enough in the (at times) harshly lit environment of an aircraft cockpit.
2) Their battery life is uncertain.
3) They are simply NOT reliable enough. Connectors, components and switches are all tiny and delicate and they pack in now and again and ofter without warning. For example, my current xda Exec, which is nine months old, has an intermittent fault and will periodically freeze and need to be reset. The software also, even WM 5, is typically Microsoft and will periodically need to be reset or will freeze itself, etc. Think Windows 98.
Sorry, I disagree.
Having used a number of different dedicated aviation GPS products and PDAs I would make the following comments.
1. The screens on the better PDAs are brighter than on all GPSs and the definition is better,
2. Battery life is not uncertain, and the gauge on the pda provides a good guide to the rate of consumption. More to the point with the iPAQ I use with the extended battery and a full charge I know the unit will provide at least three hours use solely on batteries. None of the colour moving map GPSs that I know of will do that. Of course I agree for absolute relability it is preferable to connect both a GPS or iPAQ to the ships supply.
3. I would accept that some PDAs may not prove stable. The answer is simple. Read and follow the recommendations on the PocketFMS forum. I use an HP, both in the car and in the aircraft (if I am not flying with a panel mounted unit). I have never had the unit fail.
4. PocketFMS is not complex but there is a little more to it than simply plugging in a dedicated aviation GPS. However if you have a computer connected to the internet via broadband and are happy to cradle the PDA to the computer you will nott go far wrong.
5. Finally PDAs offer touch screen features - they are useful and you wont find that in many dedicated aviation units.
it's easy to turn the unit on by accident
Very true, and definitely a trap to be aware of.
The iPAQ comes with a slide "on off" switch which disconnects the battery from the unit so even if you should push the on off button by mistake or re-insert an SD card that has come out the unit will not turn on.