Apparently, there's no cons, but I'm sure there's some bugs.
Have you used it in the past? I guess it's a great tool for mass emailing, but some organizations have different policies on third party tools. I think the best reason to use it is that it automates the mass emailing PLUS personalizes it with the recipients information.
I've not used it before and we haven't starting using it here, yet. I realized that it's from the same people who make ZenCart, which we are currently using and that that software has a built in mailing function, so we might not use SuperMailer.