I thought that nearly all of the flashbacks were excellent. The Aragorn/Arwen ones are fine so long as you don't think about them too much

and I
loved the flashback telling how Smeagol became Gollum.
I think it was necessary to include flashbacks, simply because of the depth of the story. It would not have made sense to have people explaining things to no end when they can be seen in a tidy little flashback scene.