I saw this today.
Three quarters of the way through the film, I thought it was a shoo-in for my Top 5 this year.
Then the ending came. No, not so much the big twist-- I'm still not sure how I fel about that-- but the obnoxious voice-over narration that tells us what the movie is all about.
So, all in all, I give it a B+. Allen is indeed great, and Costner is even better. The writing is mostly superb. It's funny, poignant, insightful, sharply observed, well crafted... and it all crashes into a rather clumsy ending. But still a delight.
From what I remember, I pretty much agree with what you said.