The D800 is a very nice camera for video. Not as nice as some other current options, but if you already have it at hand (for stills, I guess), it makes a lot of sense to use it.
The biggest downside is aliasing/moire, and you can solve that by adding a D5200 to your arsenal.
Of course a C100 or FS700 would be better, but that's a lot more cash too.