No, haven't seen those. Usually fairytale stuff ends up being kind of cheesy and I sorta figured those would be.
I've been watching them, and yeah, they're nothing special. So far, Grimm goes like this, every time: A crime happens that involves some creature related to some fairy tale. They look human to people, except this cop whose name is Grimm. Yep, he is related to the fairytale guys who wrote the stories, the premise being that the stories were not so fictional after all. He somehow just happens to get involved with every one of these cases, which happen week after week in his jurisdiction. No, not cheesy at all.
Once Upon a Time is more like a saga that in some ways reminds me of other supernatural-tinged stories, like all the vampire crap. The protagonists right now seem to be Snow White and Prince Charming, except that they're in the modern world and don't know who they are. Some curse through the centuries from some witch is involved. Wonderful excuse to stretch out the storyline as long as the ratings promote it.