Double-check the label but the "ready-to-drink" Crystal Light is made with Splenda (sucralose) and is fine on all phases of Atkins. The mix-it-yourself has Aspartame and it is recommended you consume as little as possile.
Now the bad news
It HAS to be plain water for the 8 cups a day. I still have the actual medical article kicking around somewhere but here is the "back of the napkin" version:
The liver will spend it's "down time" producing chemicals critical to the breakdown of fat. As soon as something besides H2O is present in the liquid you drink, the liver pitches in to aid in the breakdown/digestion of this liquid and fat conversion goes on hold. And of course the quantity aids in all kinds of well-known good stuff. That's just one of those things there is no way around.