Whether you like it or not, legally a GPS is a backup to your primary navigation
I'd like to see a reference for that, in the primary legislation (the UK ANO)
I agree that PDAs are too flimsy in terms of software reliability to be useful as one's main (or only) GPS. The battery life is not only uncertain; it's easy to turn the unit on by accident (e.g. by accidentally knocking a plugged-in SD card) and then you find it is half flat, or totally flat. I've spent loads of time playing with PDAs and would now buy a dedicated aviation GPS.
Tablet computers are a lot better - still Windows reliability but XP is a lot better than pocket/pc - but much more pricey especially if you want the special polarised screen necessary for direct sunlight viewing. I use the Motion LS800 and it works well, but needs an expensive flash hard drive replacement above about 13000ft.
If I was an average mostly-VFR pilot I would buy the Garmin 296 or 496. There are better units, e.g. the Avmap EKP IV but on usable battery life the Garmins win.