If you ask for the lighter life official line, it will always be a 'no' cos they want you to buy their products! I have a friend who's lost about 5 stone and she's always used the marigold stuff, so I totally believe it's fine. There was a thread a while ago about coke zero also. I asked my counsellor about this and again she said the official line is no, (surprise, surprise!) however, it has no citric acid which is the problem substance so she couldn't really see a problem with it. I have a bottle a day and look forward to it.
My cousellor says it's fine to drink all of your 4 litres with flavour - she even reckons the savoury soup can count towards your water. I drink all of mine with half hot and half cold water in a pint glass and either st. Clements or fruits of the forest and it's soooo much easier to drink! (reminds me of hot ribena we used to drink when I was a kid!)