With me the effect of negative ads is inversely proportional to the level of the race.
With a president, for instance, you're looking at a multi-billion dollar election cycle that involves both the candidates spending tens if not hundreds of millions of dollars getting their/name/message/brand out there, the parties spending hundreds of millions more to back their guy, PACs running ads both pro and con, the Commission on Presidential Debates doing their thing, and media of all sorts spending hundreds of millions, if not billions more, drilling down in to the candidates to find what's really there.
Plus you're generally looking at folks who have long careers in politics or industry and accessible records which are demonstrative of the policy positions they take and the politics they employ and personal histories that help you get a feel for the candidates' character.
I don't want or need negative campaign ads in such a case and to the small extent that I'm exposed to them the charges they make are usually pretty easy to substantiate or debunk.
Where I think negative ads probably have more of an effect is all the way down the government ladder at the municipal level.
If Candidate A sends me a glossy brochure explaining how Candidate B has been involved in, say, an effort to grant additional local land use to a mining company that does a lot of blasting in town and has been offering them a 20 year tax abatement on the use of that land in order to bring a half dozen more jobs in to the community, I'd probably actually take notice of something like that and consider it when casting my vote.
I don't like the blasting that currently goes on and I don't think our community needs six new jobs which, effectively, would be paid for by the current citizens of the town through deferred tax revenue through a point in time well past the point where I expect I will have sold my current home in this community and moved elsewhere.
So a bug has been planted in my head that sort of turns me off to Candidate B but the race is at such a low level and the candidates are, relatively speaking, so unknown and so campaign-fund-poor that it's unlikely I'll ever hear a rebuttal from Candidate B or have access through research to the kind of information I'd need to debunk the claims on my own.
In the case of a president/Senator/governor negative ads are a waste but in a municipal election they could be a game changer.