I got my Surface Pro 3 I7 512GB on August 2nd. So far it's been pretty good.
I got the Ghost Glass from MIME for my surface. No bubbles and you cant even tell it's there so having said that, it's awesome. Works perfectly fine with the SP3 pen exactly like without the glass protector.
Overall my experience has been mostly positive. I haven't run any stress tests or bench marked it at all but I do have bitlocker enabled and the performance has been great but I am a pretty light user so I'm not really pushing it too hard. I use my gaming laptop for gaming (ASUS G750jh) as I wouldn't expect too much from a SP3 for gaming. My office provided me a Dell E7440 which has very similar specs as the SP3 and I wouldn't play games on it either. Little games like Microsoft Mahjong would be fine but the big FPS games I wouldn't bother. That's just me though.
It did get kinda hot in the upper right corner of the back and the fan came on when I did the windows updates and firmware updates but it stays cool and quiet the rest of the time. Same deal on my Dell E7440.
When I install firmware updates, I've been running into a problem where it gets stuck on "Please wait while we install a system update".
Check out this post.
I think the cameras both front and back really suck. The quality is terrible for such a premium device.
I haven't had any problems with WIFI. Others have said that it's an issue for them. I'm running it on Wireless N so maybe it's just a Wireless AC issue. Not really sure on that.
Otherwise, mostly positive experience here.