After having some time to play with the Trigger Grip I have some initial findings...
Firstly I have large hands, all gloves have to be XL and Large size does not fit me... I find the trigger grip looks the part, and I really like how I no longer have to make sure the surface I place it on is free from anything that could scratch (beleive me when you have a 3year old in the house, you would be amazed the stuff you find laying around) it is not too weighty and can best be described as it makes you feel like you are holding a PS3 controller...
I did have a couple of issues... the volume button is not easy to get to on the fly, neither is the power, they require me now to turn the vita to be able to press them, the trigger grips feel a little stiff, they may loosen over time I suspect, however on Unit 13 you use both controllers, I found the back trigger for the crosshair aiming a little uncomfortable to reach, it almost felt like the side handles needed to be about an inch longer to allow that natural grip.
on Motorstorm RC where you dont need the trigger buttons I found this grip really shined, suddenly the comfort levels shot through the roof, which is a bit strange as I suspect this grips strong point is aimed at FPS, playing Unit 13 was certainly more comfortable with the grip tham without, but not as game changing as I expected it to be... I really noticed the benefits of it when I was sitting down, as when I stand up the Vita seems a natural hold, but sitting or laying in bed I do find the Vita starts to get uncomfortable after about an hour.
Hope this helps, it cost me £17.32 from Amazon.com shipped Via UPS.