I don't mind clones, when it makes sense for the character, and it's a worthwhile addition, they're a ****ing perfect way to add an extra character "for free" (as in, they're less work both to implement and balance than anyone else). Basically, if they actually work better as clones/semi-clones than they would as as completely unique characters because of how similar they are canonically, they're good clones in my book. Also, when they're there because they "deserve" being in the game rather than getting there purely because of their clone-y status.
I ****ing hate clones that don't make sense, or are evidently just filler.
Falco is a good clone. Dude's a part of the same team as Fox, so it makes sense that he has the same gear and fighting style. But that's not just everything, he actually gives his own spin to the style meaning he actually feels different to play as, he has a different rhythm, and especially in Brawl, a lot of moves work differently, even while they're still similar enough to Fox's so you can still see some similarities.
Luigi is a good clone too, he's Mario's brother, and already has the same abilities as Mario in their own game. But again, both in Smash and their own series, while having the same abilities, they each have different quirks.
Now take stuff like Ganondorf. Clones like Ganondorf are freaking harmful to both the game and the character. It makes it seem like they're not even trying and comes across as lazy, (even when they give him custom animations so it's technically the same amount of work) it doesn't make sense in that Captain Falcon and Ganon have nothing to do with eachother whatsoever, and in the end, it's a problem because Ganondorf doesn't end up with any of the abilities he uses in his games.
I also hate clones like Dr. Mario, because even though they play slightly different to their counterpart, they're not a separate persona. It doesn't matter how you put it, Dr. Mario is Mario with Dr clothing, and honestly, not even an original moveset would change that. Also Pichu, 'cause anyone could say that the only reason Pichu ever got there is because it can use Pikachu as a base, if Pikachu wasn't the most popular Pokémon out there and wasn't in the game, Pichu would never have made it either.
Lucina? I have mixed feelings on her.
I think it works, she looks similar to Marth, and the very little I have played of FE: Awakening (should really play some more of that thing sometime) tells me that it makes freaking sense that she moves the same as him. Not to mention that she's ****ing popular, Chrom and Robin may have been the main characters in Awakening, and that's why pretty much everyone was expecting one of them rather than her, but from what I've recently learned, Lucina is actually more popular and generally liked than either of themm so it really seems like an overall good addition to the roster even beyond her clone status.
Except... I've been reading on comments on FE fans that it doesn't actually make sense once you get past the first few chapters of the game, and that Lucina actually has absolutely nothing to do with Marth beyond the fact that she impersonated him, and deserves her own moveset on her own just as much as Ike does.
So I really don't know what to think of her.
Either way, I'll just say that the only Brawl "clone" that I have any problems with is Ganondorf, and that I don't see characters who are commonly see even as clones as "semi-clones" (meaning Lucas and Wolf)
That will tell quite a bit on my stance on clones, I think.