I dunno...it really depends for me.
I'm not huge on gimmicks in the first place, but some of them make the character really interesting. However, it's really easy to go overboard and turn people off, especially if you don't know the source material all that well.
Case in point:
's mechanic was 100% cool with me (I even liked Zelda's transformation) since it matches so well with the source. I know a lot of people
really hate it though. Even the novelty is wearing off for me - I liked how the mons were treated in Brawl better, where they each had their own name and the stock icons in the final screen showed who exactly got KOd. Tangent: I'd prefer the ability to choose to play all three or one mon only (so Squirtle or whoever is always out) with a toggle on the CSS, and then the announcer could give a "Squirtle Wins!" at the end. That'd also be great with amiibo, since all three mons have their own, plus the trainer for the switching playstyle.
's mechanic is also fun, even though I know nothing about the FE series. It adequately shows equipment damage over time and conservation of resources without being too obstructive.
's mechanics also really show off their source material without being inhibitory, and it makes them interesting and fun to play...if you're into them.
...but...
I can't play
to save my life, and he's only got one gimmicky move. I know less than nothing about the source, so it doesn't have a 'wow' factor for me, and I'm just not a fan of the character in the first place. I just feel that juggling five different buffs in the middle of a free-for-all is too confusing. And then they added the DQ Hero, who I have just as much relationship with but now has about 20 or so potential move choices? I couldn't do five!
All in all, I think that the gimmicks can make characters more interesting and well-rounded, but they can also turn people off as well. I think we should get a mixture - gimmicky characters for the people who really like them, and also basic characters who will appeal to a wider group. There's even something to be said for characters with minor gimmicks that are easy to work around.