I have played with mine now for several weeks and it is, quite simply, awesome. I do not have a sophisticated enough graphics card or router to enjoy all the functionality of this tablet but I am still VERY impressed with it.
To begin with, it is running nearly-vanilla Android. There is no skin and only a few added apps, not what I would even call bloatware. It comes with one added game, Trine 2, which isn't really my kind of game, but it is the full game, not a demo trying to tempt you into spending more money. It also comes with an app for either mirroring the screen via HDMI at 720p or going into a console mode at 1080p but I think that requires the high-end, dual-band router, which I don't have. It also comes with the Shield Hub which is the "store" showing what apps have been optimized for the Tegra K1 processor. There is also an app which, assuming you have the appropriate router, high-enough transfer speed, and low enough ping, you can stream games from their California-based server but this service is still in beta testing.
There are a couple extra option menus in the settings and I think it's pretty spanking awesome that you have clocking options, built right in. I've got it set on a balanced mode that is a nice compromise between battery and performance.
I have not purchased the magnetic cover or the wireless controller yet but I did buy a wired X-box 360 controller from a Chinese website for $20 shipped. I have been very pleasantly surprised with how well it works through a USB OTG adapter. I played Bard's Tale from start to finish with the controller and loved it. It's so much easier than on-screen controls. I gave up playing Bloodstroke before the controller arrived but have quite enjoyed the game now that I don't have to settle for on-screen controls. One advantage to the wireless controller, though, is that you can charge the tablet while using the wireless controller where I am forced to run on battery with the wired controller.
That being said, though, I get a solid 8 hours or so while using the wired controller through the USB port, mirroring the screen through the mini-HDMI port with wifi on. I think that's pretty impressive for a single-battery tablet considering the tablet screen is on while using the mirror mode, especially considering it is running the currently fastest processor out there.
I rooted it quickly and very easily with the Towel Root app, per the recommendation
here and it worked as advertised. 30 seconds later I had adblock installed, downloaded the host files, and was ad-free on the tablet.
nVidia claims they will be very actively updating the OS as they don't have to add much to the Android source to bring it to their Shield devices. With the processing power of the K1 chip, I expect this will be up to par for several years. My previous tablet was an OG Transformer, TF101 with a Tegra 2 processor. Even with overclocking, it became pretty laggy after about two years, even with updated ROMs and resets. I expect this tablet will have much more staying power unless Android shifts over to 64bit and really ups the processing power needed.
The stylus isn't quite as nice as the Samsung S-pen but that is largely because it is a passive, capacitive stylus rather than a WACOM digital stylus as the Sammy pens are. It is quite nice that it has a very small, chisel tip rather than the hemispherical tips on most styli. It allows for much more precise positioning than a standard stylus and the soft, chisel tip allows for multi-pressure usage as more of the chisel tip comes into contact with the screen the harder you press. It's actually quite ingenious. My only complaint with this passive stylus is that it is so precise, I frequently change screens when I'm trying to tap an icon because it slides just enough on the screen to slide the screen rather than activate the app I want.
The two things I really disliked about the cover that nVidia sells (didn't buy one, only looked online) are, first of all, the price, and secondly, there is no drop protection for the corners or back of the tablet, only the surface of the screen. It's very cool that the cover "snaps" on via magnets (love that about the MS Surface keyboards) but I want
some drop protection. I've ordered a case/cover from Amazon that combines a plastic snap-on case that covers the back and corners of the tablet and has a similarly-folding (and hopefully magnetic) cover like the official one. I'll report back after it arrives and I've had a chance to check it out. It appears that the aftermarket accessories are beginning to show up.
All in all, you don't have to be an avid gamer to love this tablet. It is not significantly more expensive than a Nexus tablet but you get a micro SD-card slot, nearly vanilla Android experience, built-in clocking capabilities, easy rooting, and great battery life. What more do you really want in a tablet??
I hope this will be of service to anyone considering the Shield Tablet.