KC, I think it's a matter of horses for courses ...
1. The PDA solution with MM Navigator is not really a GPS unit solution ; it's a moving map. However, coupled with the CAA digital charts, VFR nav - in the sense of "where am I exactly?" - becomes very easy, particularly in respect of airspace boundaries.
2. You most certainly can use it for "instant reference" flight planning ; you do the planning on the ground on a PC, and then just use the PDA for a visual check in the air (backed up of course by other, traditional means)
3. As to cost benefit, it's not something I've really thought about greatly, as I use the PDA for many other purposes as well. I also have a fairly well equipped plane in any case, so I'm not worried about it not having an HSI style interface, or "GoTo" functions etc - I've got those covered anyway.
The only thing which I'm having difficulty with is getting digitised versions of non-UK airspace maps, which I guess the traditional GPS route has the drop on, but I'll get there with that eventually.
FF