OK, so I've had some more time with the Shield.
This thing is absolutely, astonishingly fun. Shadowgun and Shadowgun: Deadzone are incredible on the Shield. This is the way those games were meant to be played. I can say the same about the stuff by Vector Unit (Riptide GP, GP2, Shine Runner, Beach Buggy Blitz). On the Shield, these games are just plain FUN and effortless to play. The Tegra optimizations make them even better.
I have had (and will have) zero experience with PC streaming. I bought the Shield as a mobile Android gaming system. As that, it excels. Hands down the best Android gaming experience thus far. (Even beats the Ouya... ahem.)
I experimented with some regular apps, too. Facebook, Plume... both work as expected. It's actually very easy to use the Shield as a social media consumption device. It's not as intuitive to actually post items, but the onscreen keyboard works fine. It's a bit awkward, of course, but it works.
The Shield works great as a video-viewing device, too. It wouldn't be my first choice due to the screen size, but the *quality* of the screen is outstanding.
I could go on and on, but I feel like I'm just stating what others have mentioned previously.
I am very pleased with my purchase. Yeah, it's too damn expensive. But it's soooo nice. If it were $100 cheaper, it'd be a no-brainer.