The input lag is actually fairly bearable in the recently updated game mode for development and everyday use, though I certainly wouldn't use it for competitive gaming.
My main issue with it as a monitor is the Automatic Brightness Limiter. Rtings has a fairly good description of the symptoms: http://www.rtings.com/tv/reviews/lg/c6
Basically, the brightness of the TV varies across a huge range depending on the content being displayed, with brighter content causing the TV to dim itself automatically. It's a huge distraction on a day to day basis because every time you switch to a new tab in a browser or minimize/maximize a tab, you can see your entire screen dim/brighten. There is no way to turn it off completely AFAIK, even through the TV's service menus. The workaround I've found involves setting the contrast level to 55 or below, which reduces the max brightness of the TV and makes the ABL changes much less drastic, but it's definitely still noticeable and distracting, and of course the contrast and max brightness suffers.
If I had known everything about this TV that I know today, I would not have bought it to use as a monitor (would probably have gone with some high end LCD with good local dimming). Watching movies on it in a dark room is an awe-inspiring experience though, and is what makes me reluctant to go through the trouble of returning/selling it and getting something else. That's my 2c anyways, feel free to make use of this information how you will.