I usually have to try a couple times before I actually succeed in pressing it. A little annoying, but no biggie IMO.
I think it would be better if it was on the back slope of the edge. Right now if you want to push it using the hand that is holding the phone, your finger has to curve around and over the top of the phone, which is an unnatural movement.
My phone progression is identical to yours.
I like the buttons... and it does take me multiple tries at times, but that's fine. It's better than taking my phone out of my pockets and seeing the screen was on or the camera was activated.