I had pre-ordered this tablet, so I'm one of the earliest adopters. Here's my experience.
The USB port is loose for everyone, but I've had no problems, and I even dropped it once right on the port with a cable plugged in (to be fair, I consider myself extremely lucky). I've also had no issues with my power button. As far as "how reliable" it is, that's a pretty vague question, so I'm not sure how to answer that. It went to work with me every day for quite a while, and I use it all the time for gaming, be it hooked up to the TV or just used as a tablet. I've basically used this thing for everything it can be used for, and it's still working great.
My only complaint is that with the release of the Shield Tablet, it seems that Nvidia is playing a little bit of the planned-obsolescence game by not bringing certain software features to the TN7 that it is clearly capable of, primarily Console Mode, which is supported on both the Shield Portable and the new Shield Tablet, yet the TN7 still doesn't have this feature (and it's one of the reasons I bought this device... so much for my false expectations). I find it infuriating when any company purposely withholds features from a previous product just to try to strong-arm you into buying their newest product.