Let me warned you that while parts of a lot of reviews of this series does not qualified as spoiler, I think most people can guess just by reading the non-spoiler part of the reviews what will happen, mine included. So let me warn if I you happen to read this first.
Personally, I like the story through and through, from the beginning to the end. I respect and understand why many people have problems with it, but I stand by my opinion. Yes, it could be another type of ending, but this one is also reasonable. I don't think it's sudden either. The author actually spend time to show the development, but if you marathon through the series in one go, you probably feel it is sudden.
Also, I think Daikichi's respone to the fact that Rin like him is the most reasonable one that I can think of. He gives his objective opinion why he don't really like that, from the parent point of view. When pushed, he still hold his ground and tell her to wait and see for another two years. It shows that he takes her feeling seriously, but also not forget the fact that she is young. It might just be a short time thing. Still, after two years, Rin proved herself that she is serious, and her feeling lasted. You have to remember that there is another two years time skip at the last chapter. They both wait and think about it for quite a long time before actually change their relationship.
Unlike others, I actually think that the ending is not cliché.
Actually, ending up with Kouki would be cliché in my opinion because that is what I thought almost the whole time up until after the development of why Rin stop liking Kouki (but still care about him like a brother).
In the end, I highly recommended Usagi Drop. The whole story of course. Sometime there is a story that you like but you just prefer another type of ending. This maybe one for many people. Example of that for me is Toradora! Even then, I don't think you should stop (or suggest others to stop) reading in the middle and miss another good part of the story just because you don't like the ending. On the other hand, if you're like me, who like both the journey and the end, you'll be reading this over and over. Go experience the whole ride, and reach your own conclusion with the whole picture. Happy reading.