I do get quite a few emails from such companies, however, I am always sceptical.
Most are based miles away from my area, and as such I find it hard to believe they can drum up business in my local area.
I would avoid such companies, you are not providing a "niche" service such that you will necessarily attract clients outside your catchment area.
Furthermore, most of these companies will want you to sign up for a certain time period, and you will end up paying for perhaps little or no business at all.
I would say avoid like a plague.