I just replayed it (Wii game) and I do realize how long it is. They could have cut out an entire section of it and it would still be like 12 hours long. This type of game isn't usually 20 hours. I don't think it's too long, though by the end the mechanic is wearing a bit thin. I think it's evident in the designers making interesting bosses to mix things up. The last "traditional" boss is Salazar, I think. It, Krauser, and Saddler are all pretty interesting boss fights.
I certainly don't think it's the second best game of all time, but I don't think it's a C level game either. It's an A, and an A with high replayability. I'm shocked there haven't been more RE4 clones.