I'm going to have to flip my opinion of these two shows after seven episodes of each. Grimm has turned out to be a much, much better show.
Though I found the first episode of Grimm awkward, the writers seem to have really grown into the characters and smoothed them out. The complications of the main character being both a Grimm and a detective for the police has turned out to be really interesting and kind of hilarious. I can just imagine the evil beings of the world mocking him about killing them and then have him turn around and be like "Kill? Nah. Enjoy prison jackass!" His reluctant werewolf not quite friendship has also been a lot of fun. The werewolf clearly does not give a crap about helping him but is easily talked into it because he doesn't want to end up on the bad end of a Grimm knife...or in an interrogation room. They have done a great job keeping that relationship casual with tense overtones and very funny interactions. That was something I also didn't expect from the show - it has a Castle like humor between the main police officer buddies that I love. If you haven't watched it yet, I would definitely recommend popping on a few episodes.
As for Once Upon a Time, that show decided to go completely rogue on me. The first episode was enticing, but the more I watch the more I realize Grimm's one-off easy to watch alone episodes are much preferred to Once Upon a Time's very obviously never going to be resolved plot. I thought the show was going to be a sort of lighthearted fun and go off the more modern happier fairy tale versions of Grimm tales, rather than the darker ones. Boy was I wrong. My initial hesitation with this show was that they portrayed the real world as a "place with no happy endings" but then the fairy tale world they show flashbacks to is WORSE than the real world! It sucked there! Rumpelstiltskin pretty much screwed up all their lives in that time where the evil queen is messing with them in the new era.
That being said, I am extremely tired of everyone painting Rumpelstiltskin as a bad guy. In the real world he is not that bad at all. They tried to establish him as evil in the first episode but it was half hearted at best. The only other time he was remotely evil was the Cinderella episode, and my god, Cinderella was horrible in both the fairy tale and real worlds. I did not feel bad for her at all. She was a selfish brat and a complete moron in both versions of herself. I had zero sympathy for that character. I am having a hard time caring about any of the characters.
Then there was the Huntsman episode, and if you've seen it, you know what I mean. The plot finally progresses a little, we get some real evidence that their world might be "saved"/it's not really all in the kid's head, and then in the next instant they destroy it. So fairy tale world is back to square one. I really don't have much interest in watching the poorly written characters fight and scrape their way back to another small win.
If you don't mind a major spoiler about the last episode TV Addict has a fantastic article about why Once Upon a Time is dropping like a rock in ratings that I completely agree with.