I tend to afford caffeine now, but I went out to the pub recently and they didn't have any soda water or diet drinks (great pub lol) so had to have normal coke. It made extremely hyper active, I was talking for England and my mate asked me if I had been taking drugs!!
Having been a caffeine lover in the past I was pretty surprised at the effect it had on me. Must of been all the sugar and caffeine that made me hyper. It didn't stall me either, but I did only have the one drink oh and it didn't kick me out of ketosis
As long as you don't have it all the time then you should be ok