I've been using the K1 for a bit more than a month. I really haven't had much trouble with it. I've run their 4.4.2, 5.0.1, 5.1.1, and 6.0 releases on it. The problems I've had are primarily with apps not working on all OS versions and this is not really a NV specific problem.
The initial 6.0 had some quirks like the touchpad on their controller was non-functional. Yeah I have the controller. It's interesting and works well, including the gamepad mapper. There is also the apparent problem with the logd daemon hitting the CPU too much and wasting battery. I have not tried the newest 6.0 release. I'm waiting on NV to put it into the OTA system again. I'm not much impressed with 6.0 though because it's a lot like 5.x but adopted storage is not so hot and I preferred the old "move to SD" function (which is gone).
I own/have owned a bunch of different tablets and am used to fighting with Android quirks. I tend to go for fun projects like the old Nook tablets, some Chinese tablets, and Windows tablets. At the moment I have an Onda V919 Dual OS, Tegra Note 7, Venue 8 Pro, and the Shield K1. Samsung might be the way to go if you want the most solid experience. I would read the forums first on that though to be sure you aren't dreaming.