We have been to both Garden Grill and Chef Mickey's multiple times, and absolutely prefer Garden Grill. It's Mickey, Pluto, Chip and Dale for dinner. It's our arrival night tradition every trip, and it's been our boys' favorite restaurant since each was little (our oldest's first trip was when he was 18 months old.) Our boys are now ages pre-school to pre-teen and it's still at the top of their lists. As others have said, it's some of the best character interaction at Disney. The characters always spend lots of time with us - we never felt rushed by them - and we usually see them each at least twice per meal. Last time, our oldest forgot his autograph book in our youngest's stroller outside the Land, and Chip came back an extra time at the end of the meal looking for our son and gesturing about his autograph book (which he had retrieved and got the signature!)
Garden Grill is way more relaxing to us than Chef Mickey's because: 1) the booths on the lower level keep you from seeing all the other activity in the restaurant so it seems like just our family, the characters, and whatever scene from The Land ride that is passing by at the moment; 2) family style service instead of buffet means we don't have to juggle plates with the kids, take turns going to the buffet or possibly missing a character when he/she comes by the table.
We think the food is much better at Garden Grill, although it's not the best food ever, it's good. It includes healthy options and lots of kids' items - so it's a good amount of food and a nice variety.
That said, we still go to Chef Mickey's once in a while. It's classic Disney, it includes Minnie (GG does not,) and it's fun to have dinner at CR with the monorail. In my opinion, the food is OK - I've never had the really horrible experiences that others post about. But if I had to pick just one, it's definitely Garden Grill.