I like mine. Think it is the 778. I opted not to get the GPS but rather get a handheld GPS for versatility - ie: in the canoe, hiking, etc. Anyway, I'm quite pleased with this setup.
The 5-- series would have done just as well for me, but mine was a good deal on a previous year model. The 7-- series had some additional features, namely around connectivity - to a downrigger, to another sonar and mine allows for connection to external GPS. I need none of these features though.
The screen is very high-res and shows lots of detail. When jigging, I can actually follow the jig and can even see line on the screen.