I've got some lovely chunky antiqued-bronze zippers that I will be using on my daughter's outfit (if I can find what I've done with the blessed things) along her lower sleeves and the front of the bodice in place of buttons. I think it will "punk out" an otherwise reasonably traditional design quite nicely.
For me, I think they can be really cool as a design element. But I'd say that, in contrast, an otherwise Victorian dress with a plastic, same-colour zipper in the back instead of the appropriate buttons or laces would end up making the outfit look slightly cheaper, rather than Steamier. But that's just me.
Me, what I particularly hate is otherwise fairly decent Victorian-inspired lace-up boots with that damned zipper down the inside. Not only do I really dislike the look (it just being a "convenient" way to have to not deal with laces, so I kind of don't like the attitude I personally find implicit behind them, that of laces being somehow too complicated or fiddly to bother with), I find that boots with these zippers never seem to want to lace tight to the leg. There's always a gap at least large enough to fit a finger into, and that throws the line off, for me.
I have a really lovely set of granny boots made of lovely soft leather, with pointy toes and a thoroughly appropriate heel, and nary a zipper in sight, but my bloody dog chewed on the toe of one.
I do have another set I use now, sort of a slightly delicate heeled hiking boot that I think works well for the persona, but they have that damned zipper in there again, which means that if I wear spats they won't fit right.
--Sorry, got distracted there. I guess the tl;dr version is "Depends?"