I personally own a Tegra Note, which is the Shield's predecessor. I quite like it and it performs well for gaming.
Only complaints I have are that the GPS does not function well (it takes 15+ minutes to acquire a lock), the battery life is rather short when gaming, and the cover for it (which seems to be about the only cover/case available) is cheaply made and fragile.
But I don't use the GPS on it (I use my phone for that), I can just use an external battery to keep it charged, and I gave up on any sort of case for it.
Another device you might look at is the
Wikipad. It's a NVIDIA-based 7" tablet, but it also slides into a controller housing you see it in in the pictures making it functional for portable gaming, like an upsized gamegear, rather than having to put the tablet down on something like with the shield.