Just played the Vita in two stores, WipEout first ofcourse :^) For me the fluidity of the display because of the total lack of 'motion-smearing' or ghosting was the most impressive, the framerate was indeed rock-solid, very good. The OLED display looks really vivid. WipEout looked good and the controls felt tight. The device was hefty but comfortable to hold (I'm used to the Atari Lynx, so I don't mind).
Loading times were pretty bad for a handheld with all of the games I tried. The mini-analog-sticks had a lot less resistance than on the DS3 and a lot less range ofcourse, being so small. I'm not sure how precise they will be in practice.
The action buttons were tiny, felt even smaller than the PSP-Go. Shoulder buttons were the same as PSP slim. Back touch takes getting used to. I played a greyscale game, not sure what it was called, where you used front and back touch and I really had to adjust to it.
Motion controls looked more precise than I'm used to, my phone and the sixaxis are pretty jittery by comparison.
I also tried Ridge Racer, which was just that, not really notable or impressive looking. Super Stardust looked really sharp, almost 3D with amazing colors and used shaking for smart-bombs. Gravity Daze (or whatever it is called now) looked cool, especially the amazing looking animation of the character's hair. Controls took a bit of getting used to and I personally don't really care for the cartoon outline effects. Had a quick look at Hustle Kings, which looked similar to the PS3 version, so quite nice.
The menus were smooth and responsive, the menu buttons look like shiny Mentos.
There was someone from Sony who brought his own Vita and the round metallic buttons on the top of the device, together with the memory and cartridge covers looked really cool. The Sony representatives made sure that people understood this was not the PSP and different from gaming on a phone. Ofcourse the Vita isn't as revolutionary as the PSP was, so I hope it will succeed despite that. Interest in it was okay, but no crowds, I had no trouble trying it as long as I wanted to.
The much improved display is the star of the show, vidid, smooth and detailed. I did miss stereo-3D a bit, but it would have had to be better than the too narrow sweet-spot of the 3DS.