I've been using Maptech's Pocket Navigator for 3 years. I bought as an interim solution and emergency backup to paper plotting, but I've found it entirely adequate, and decided to stick with it rather than get anything better. This is because a) sharing one set of charts between PC at home and PDA on the boat is economic, b) Maptech are gradually adding the features of the high-end plotting software eg. weather overlays, tidal routing, AIS plotting and c) I'm trying to avoid having a PC on the boat. The main upside of Maptech to me is that it's very straightforward to use. The main downsides IMO are a) their charts are now quite expensive compared to the latest 'wide' format vector charts, if you want to cover a larger area, and b) it only supports a limited range of raster chart formats.
There's a version of C-Map called 'C-eMap' for PDA, which comes on an SD card and has wide coverage, but I haven't seen it in the flesh - it looks interesting as I've come to like C-Map charts (I used to hate the way the text was placed indiscriminately on the chart), but I wonder what features they've left off?