Yeah, this seems to be a case of diminishing returns like many of the N64 ---> Cube translations, but I can't say I'm disappointed with the results, either. It's still an awfully solid game, and I don't mind the controls or the foot / tank missions at all. They add variety, even if they keep it from being the true Star Fox sequel I would wish for.
I don't think replay value will be an issue here. I've only done 5 missions so far (just got the game today) and I am eager to replay those I've done as I am to see new ones. It seems well worth the money to me, but then again maybe $40 isn't as much to me as it is to some others?