I've been running one for a few years. Not had any probs with it. Easy to use even if you don't read the instruction book. Menu driven off of the ent,clr and joystick pad thing. Use Navionics or in your case Navionics Gold carts. What sort of money is being asked for the one your looking at? I've got the previous Geonav 5 which cost just under the 130 quid mark when I bought it.
I've no experience of this model but, on paper, the Geonav 6 doesn't look to be very good value. For example, it's not waterproof, and it can only handle 20 waypoints per route. And it only has a 5.7" screen.
I've had a lot of experience with the Navman 5600 which, for less money, is waterproof (IP67), offers 50 waypoints per route, and has a bigger screen (6.4") - which is actually 25% bigger in area. The Navman is also outstandingly easy to use, with a simple intuitive menu system, and some interesting features such as the "ribbon compass" display.
However, both the Geonav and the Navman only have 320x240 pixel screens, so definition isn't too good. And at £650 ish, they're getting close to the current prices of larger-screen high definition 640x480 pixel chartplotters. The costs of course don't end with buying the plotter, there are chart cartridges to buy too, so in the overall scheme of things it might be worth spending a couple of hundred more and getting a 640x480 screen plotter.