If you're talking about playing him optimally (as good as Luigi can get, not just 'really good' play, but the best play), then it's the steepest learning curve. To play him exceptionally well, it's pretty shallow, as you can get what is essentially a back-bone of 'specific fundamentals' and abuse his stuff to no end. Only when it's not abuse-able anymore (when the meta-game develops enough that there's no 'tricks' to fall back on), is it difficult. It's easy to distinguish between good and great Luigi play, it's the last bit that makes a difference between great and fully developed Luigi play, and it's the silly intricacies that give the character dynamics in play. As Kink kind of described, it's not 'normal' smash, due to lack of conventional... anything. When WD Forward > Pivot F-Tilt is essentially more Bread-and-Butter the more the meta-game with him develops, you know they're a ****ed up character. This also leads to why most players can jump around to many characters, but everyone immediately will adapt the same play-style when initially grabbing Luigi. So far, there are some 'great' Luigi players (Melee too), but very few/if any, people playing the character with even a tap into fully developing the character.
Otherwise, yeah, he's good in PM 2.6.
Dedede though...
He's the EXTREME of tiny differences. All Dedede's will look the same until you get to that fully developed level of the meta-game, where the dynamics of the character are so flushed out for the player, than they don't have any fall-backs, they're just a giant bag of presence for the opponent since since they have so many tricks to limit the opponent from abusing Dedede, but lack anything beyond them to make much happen. Stale-mates with instant-kills are inevitable with that character's meta-game. haha