Having the CAA charts (both half and quarter mil) on your PDA does work well. It is great for knowing exactly where you are and the software is very easy to use in this respect. In fact, you don't really interact with it at all (which is why it works so well) - you just look at the screen and it shows you the map around you.
It isn't so good for navigation in my experience. You can use the PDA or desktop to plot a route and it'll give you a heading to fly, but it won't give you a correction to your current heading, an ETA, your current position relative to a beacon or what have you, all of which you'd get from something like a Garmin Pilot III.
The Navman sleeve GPS is less than ideal in that the GPS reception is then a function of exactly where you place the PDA (on the yoke, your lap, the seat next to you or whatever), unless you use an external antenna. A very neat solution is a bluetooth PDA (the most recent iPAQ and Toshiba have them built-in) with a bluetooth GPS. It is expensive but it means there are no wires at all and the GPS can be positioned to get good reception. It's also completely portable. And for another 100 quid you can have sophisticated in-car navgiation for example (which you could with the Navman too).
The best setup I'ved tried for planes without any panel mount GPS is a Garmin aviation GPS for routing, whose signal also drives an iPAQ showing the CAA moving map.