Yeah I've a bottle of Lemon cordial (no added sugar, of course) that I use. Couldn't drink plain water all day long, and like Taz says - takes the edge of wanting sweet stuff. I've noticed no problem with this and Ketosis at all, but I like it fairly weak so don't know if that would have an effect (I drink several glasses per day).
I have noticed last week tho since I started drinking more diet coke and low cal tonic water (both containing aspartame) last week that I stopped losing weight. I don't think it kicked me out of ketosis but I think my body might have been using the energy from those ahead of my own fat so you might wanna monitor it daily (if you're that kind of person - I am!)