I didn't like he too much in the beginning (the first time that I played, in Hero Mode, when I knew he was going to chance, it was really funny). But I love "the ledgend of groose" or "grooseland" ...
He does mature... which I think most people do that have to fight against giant demonic monsters... (just something that seems to happen).
Groose was such a jerk in skyloft, I really enjoyed watching him get told off by Zelda and all the cut scenes where he plainly outright likes her. :p In the land below though, I felt kind of bad for him because he cared for that old woman so much and he had to accept that he couldn't be with Zelda and everything.. but he helped out Link so much. So I guess I like both versions for different reasons.
When I first started playing Skyward Sword, I was sure that Groose was going to be a character I disliked through the whole game. He seemed like a worse version of like "The Great Mido" from Ocarina of Time. But when he made his way to the land below he actually became one of my favorite characters of the game. He genuinely cared about the future of the world, and did his best to help Link out. He even built a freaking catapault on rails by himself. You could tell he was truly in love with Zelda, and he valued her happiness over his desire to be with her. He made a huge 180 when he came to the land below, and is probably the most redeeming characters in the Zelda series.
I like Groose later in the game because he actually becomes a huge character from that point in the game. Plus, Groose just makes me laugh because of all his "Groose" refences. Grooseland... the Groosenator, just him period make me laugh with his pompaderm.
I definitely preferred him on the surface. I liked how respectable he grew, and it all culminated when he stepped in front of Ghirahim to stop him from entering the Gate of Time. Groose had my everlasting respect after that.