I've had/played with GPS system for years, way back when we had a rather large windows laptop and a serial GPS device as big as a book plugged in.
Played with no-name brands, Tom Tom, iPaque but I consider the Garmin to be the best.
They all have pros and cons, none have ever been perfect. I like the larger screen of the 66x but I like the physical buttons of the 550. The 660 seems under powered and takes a while to draw on screen.
I have entered lots of feature request on the Garmin website, one is to do with points of intrest. If I have a route going and I want to find the nearest gas, food etc, please find the nearest on route not behind me or 5 miles off route. You can set up the gas tank limit and a warning. So I set my GPS gas tank limit to 150 and warn me when I have gone 120. The 660 will flash a warning and show a gas icon when I have only 30 miles left.
So the GPS knows I have 30 miles give or take to play with, so find me a gas station on route within my range, if it can not, then by all means go off route.
Still love my Garmin, would never leave home with out one.
Oh shameless plug, my zumo 550 with XM radio puck is for sale in the classifieds
here
Harry