I looked at considerable feedback from glider pilots who use PDA's a lot. The best ones for visibility seem to be not the latest generation but the iPAQ 38xx series - so I bought a 3850 and am quite satisfied with it. I use it with backlight on all the time. The internal battery is far too small for long flights, but there are various gismos to link it to a 12v source and step it down to 5v, one of which I use.
(In fact mine is linked to a PCEFIS, which gives a solid-state artificial horizon, and a GPS readout when coupled to a Garmin, all displayed on the PDA. Works for me. It is possible to switch off the PCEFIS and boot up nav software instead, still driving the PDA from the PCEFIS electrical source.)
Chris N.