I really enjoyed Good Grief. The characters were loving, annoying, over the top and it felt real. We all know a Sophie, Thomas and Marc. The scenery filming was magnificent too, who doesn't love Paris?! If you are looking for a rom-con, this isn't one.
I think the problem here for the film critics is that Dan has written a film that doesn't fit easily into a category, plus the audience has to really watch the film, catch the small looks, to appreciate the small details, it isn't a film to watch while scrolling on your phone. Plus in my experience, if the critics hate it, its generally a winner for me.