It made replay value feel too forced, I enjoyed that levels, but to replay a level one must do it subtly. Not "play Luigi too derp!" it made the experience feel forced.
What they should have done is the Galaxy approach, where you had a separate file for Luigi, that would give far more incentive to replay every level as him.
Outside of this flaw, I loved the game however, but it feels rushed in terms of level quantity, probably due to the very tiny team, however the Galaxy team proves they are way better at game design then the NSMB team.