>Are there any "rules" about when it is or is not good to use

Never use chocolate soy milk to make mashed potatoes. ;-) You have to
read the label and make sure you're not buying sweetened/flavored for
use in regular savory recipes. I use it to make quiches for a client
of mine and it works very well with whole eggs. If you accidentally buy
"vanilla" for a quiche it'd be way nasty.