I know this topic is old and forgotten, but I just came across this after researching Manta.com

I must say that I kinda liked using Manta, but from the SEO perspective it didn't seem to deliver much, the problem with Manta was not using it, but cancelling it.

1. You can't cancel it from the website, you need to request it
2. After I requested it, I haven't gotten a response for some time
3. After I followed up, the guy straight up lied to me that he responded already, even though there was nothing else in that thread
4. After paying for few months, I got bunch of listings online on different profile sites, what my "Manta manager" told me is that if I cancel the subscription, and I quote: "we will no longer have control over the 60+ listings and they may end up reverting back"

So in short, if you are willing to keep paying them (forever) it's a neat little tool, but if you feel like you may want to EVER cancel it, think twice, as you might end up getting an email that sounds like a blackmail.