Build quality wise it's great, the kevlar has a nice sticky feel to it so it won't slip out of your hand, and the aluminum feels very durable as well.
As far as ergonomics are concerned, the phone is horrible in my opinion. It has an odd trapezoid-type shape to it, with almost 90 degree corners and edges. For one I could never get a perfect grip on the phone that felt comfortable and natural, the most comfortable position still left me unable to use the top left corner with my thumb, and I constantly had to readjust my hand to make it feel better. This has to be one of my biggest complaints about the phone.
Also, due to the hard edges when you put it in your front pocket it looks like a deck of cards, it's extremely noticeable. Unlike most other phones like the GS3, which have tapered edges and corners which make it much more comfortable to hold, and are more discrete when in your pocket.