Fitnessqueen
The problem with soy milks is that the natural milk sugar (lactose) found in animal milk is replaced with sucrose, fructose or glucose.
Lactose is hard to digest (in fact, it's often the cause of dairy-intolerance!) and also, as it travels through the body, combines with other fats and takes them away with it.
The other sugars - particularly fructose / dextrose / glucose go straight into the bloodstream with startling efficiency (this is how "energy drinks" work!), will cause a spike in insulin, and knock you straight out of ketosis.
There are a few brands of soy drink which are not sweetened - but they are hard to find, and you may not find them as palatable.