You want to find real cherry extract. The biggest problem I see with the sodastream stuff is the artificial sweetener they put in them and the fact that almost no commercial cherry flavored things are actually flavored with anything resembling real cherries.
Not only is it going to give you a fake, jolly rancher cherry flavor, but it's going to add a fake sweetness that won't be eaten up by the yeast. Don't get me wrong, I love fake cherry. Cherry kool-aid is my second favorite thing to drink, but it doesn't sound good at all in beer.
Actually, the more I think about it, you'd probably be better dumping a packet of kool-aid in there, at least it wouldn't have that diet sweetener crap in it.