I use memory map with OS maps on a mio 550, battery life is poor, about 2 hours with the gps on.
It shows the coastline clearer than bluecharts.... beaches, PO, pubs, bay names heads etc.
Awesome on the PC as well, with some maps you get to 3D view your route, if only I was a rambler!!
But, as mentioned, no depth soundings.
The mio also runs tom tom for the car which shows petrol stations (useful for the RIB?)
Can check email using bluetooth and gprs plus browse the net in a kind of early 90's type way.
All in very useful (cannot remember exactly who recommended it but it was of rib.net
) thanks for the tip.