My first impression was "What was Ford THINKING!!" The new styling is as ugly as sin compared to the beautiful retro look of the 2005-2009 models. It no longer has a retro look outside OR inside. It is a mish-mash of curves and angles that are a far cry from the gorgeous 1969-70 Mustangs that the 2005-2009's were based on. In addition, Ford has dropped all sporty colors for the Mustang except basic red. It's either red or something Grandma would choose. I could live with red, but I hate being limited to only one color choice.
In all fairness the interior is nicer and higher quality than the previous models, but has sacrificed the retro look almost totally. The seats are comfortable, amply adjustable and, as with all pony-cars, the rear seat is suitable only for children.
I found the ride and handling little changed from my 2007. In other words, see your dentist after driving. Your fillings will likely need replacing. The cornering, if anything, felt slightly LESS stable than my 2007, but that could be attributed to the tires. Why Ford can't build these cars with a decent-riding suspension option for people who don't race (99.999% of all buyers) is beyond me.
The 4.0 V-6 provides far more than ample performance, contrary to car-nut magazine reviews, and is more than the average driver will ever dream of using in real life. In day-to-day driving you'll find yourself running up the tail-pipes of everything on the freeway entry ramps with the V-6, so why waste money on insurance, gas and unusable V-8 power?? The 4.0 is a bit coarse, and the new Fusion-based V-6 coming out in the 2011 will be a bit smoother and have even more (totally unusable) horsepower.
All-in-all, I'm disappointed. I will test drive the Camaro (also not very true to the retro look) and the Dodge Challenger (a drop-dead gorgeous, true retro style winner). Two of my former Mustang-owning friends have already traded for 2010 Challengers and are happy with their choices. I love Fords, and Ford is currently building some of the best cars on the planet. I'm just terribly disappointed to see them throw the retro look out the window and eliminate all the sporty colors.