I think Ubercake answered this the best, but here's my input.
I'm planning on buying the Nvidia SHIELD not necessarily as a gaming device, but as a super-powered android tablet. Based on the specs for the Nvidia SHIELD compared to most other flagship tablets, it seems like the SHIELD definitely holds it own and outshines most tablets for performance. With a state-of-the-art GPU, I would expect no less.
If you just use the SHIELD as a tablet, it packs a great GPU, a very good CPU, a compact design, expandable storage space, and more. The SHIELD also includes features which many other tablets lack such adjustable power saving settings, front facing speakers, a good built in heatsink, and a fine tuned stylus for drawing and writing.
The SHIELD does lack some features/qualities which I wish it had: a fingerprint scanner; waterproofing; shock resistant design; gorilla glass; and a USB 3.0 port. However, I can look past these issues because of the Nvidia SHIELD's pure power.
If you use it for gaming (like most people do), then you are going to be able to experience so many more features! The SHIELD's GPU truly does put most, if not all, tablets to shame. The gaming side of this tablet gives you access to great features including: playing in console mode on a TV; allowing up to 4 players to connect; playing intense games with high FPS and great graphics; streaming games from your PC to your SHIELD; and streaming directly to TwitchTV (if that's what you're into).
In conclusion, I think the SHIELD tablet is a great combination of a portable gaming device and an everyday tablet. The SHIELD Portable was a good first step, but it really does not allow users to use the device as an everyday tablet. With the connected controller, the SHIELD Portable is difficult to use for typing, and general navigation like you would do on a smartphone or tablet. With the SHIELD tablet, you get the look and feel of a regular tablet device, the performance of an overclocked gaming system, and the ability to use the tablet for both work and play. Coming in at only $200, I think the SHIELD is a steal compared to the other tablets out there on the market.