IMHO for road use there is no substitute for TomTom Navigator on a PDA. The voice commands are excellent as is the map particularly in 3D mode. Third party software to enable the inclusion of speed radar cameras for example is also very useful.
For the plane the decision is much more difficult. There are a few packages around like FMS. The problem is they provide little more than a moving map as useful as that can be. The downside is that they are quite difficult to actually use in the air if your detail is changing or perhaps in IMC when you don’t want to be fiddling with the interface. I personally reckon that Anywhere in terms of its concept is by far and away the best product because it is really simple to change destinations or weigh points en route, display extended centerlines so useful for accurate positioning in marginal weather and a host of other easy to use features. The downside is they have not up dated the eastern database for sometime (it is an American product) which is a great shame because otherwise it would be perfect. In fact this is not the end of the world and I still find it is the easiest of the PDA aviation products to use.
I have found the TomTom bluetooth receiver excellent for its rapid acquisition time and ability to pick up satellites even when the receiver is shielded. The absence of cables is also a real bonus.
In my view other than for occasional use any pda really needs to have some means of connecting to mains battery. The battery life is not great and if you start to “rely” on the system you will find when you get to your destination you have inadvertently left either the receiver or PDA switched on so that by the time you are ready to return home the batteries are flat!