It's a great TV not perfect but what TV is?
I don't have any issues with the brightness and the really bright highlights in HDR content is more than enough for me. Any brighter and it would start to hurt my eyes. This is an opinion shared across the owners over on AVF.
I don't have any issues with motion either, however I'm not that susceptible to it. Football and film content is as good as my previous Sony Bravia and that was one of the best in its class for motion handling. Gaming is also great although I wouldn't class myself as a hardcore gamer so I haven't noticed the slight increase in input lag over the Sony. I have read about issues some people have had with juddering on 24p content but I have yet to try any and this can be reduced using the dejudder/de blur settings.
The 3D on the C6 is off the charts! I was never a massive fan, but now will watch any film that has a 3D version. My jaw fell to the flaw when I watched Avatar and the Hobbit Trilogy recently.
The biggest downside for me is the HD content on sky can look a little soft on the lower bit rate channels. The main channels like Atlantic, Sky one, sports and the terrestrial channels all look great though.
The other possible downside is the curve. I was really worried about it as I'm not a fan but I can honestly say I don't even notice it. It neither improves or detracts from the image.
I can highly recommend it, and in my opinion it's the best bang for buck TV you can get right now. Currently a shade over £1400 from Currys with code. Oh and nearly forgot, the smart functionality is the best I have ever used and enabled me to bin off my Fire TV box.