I got to play with the GTM 650 & 750 for quite awhile at SnF on Friday...the crowds were very light. Oddly there was more activity around the Garmin Aera display that the 650/750 demos. The GTM demo's were lots of fun, and they seem to be much more intuitive than the 430/530. Many slick features, but nothing earth shattering. The screens are larger, much better color resolution, and the touch screen finger rails may be OK in turbulence, only time will tell. Flight plans, routing changes, approaches, Jepp Chartview, and airways are all quite nice.
Installation of either is not an easy slide-in swap 430/530 operation. They both require a new tray, and the 750 needs about 1.5 inches more in height than the 530. WAAS is built in, but no GPSS or synthetic vision. However, if you want to utilize more of the 650/750 functionality Garmin offers a cool built in, flip flop, voice recognition, audio panel (GMA35) and a remote transponder, which will drive the installed price of a 750 up to almost 30 grand. Chump change for Garmin.
As I've posted before very slick, but IMO not too slick. If you don't have a decent GPS, maybe you can rationalize close to 20 grand for a 750, especially if you fly lots of single pilot IFR. I, for one, would pay the premium for a 650/750 rather than installing a new 430/530, BUT I would also wait for several months so the kinks can be sorted out. Shops don't have the new boxes yet so installs won't begin for at least a few weeks. Upgrading, if you want to call it that, from a 430 or 530W, is problematic, and I wouldn't want to be a beta tester for either the box or the install right now. I suppose lots of under one hundred grand planes will be getting them soon, and that the used market prices for 430/530's will make value conscious Mooney drivers very happy.
It will be interesting to watch the new box progression after the "first kid on the block" rush subsides.