These are all great suggestions, which I appreciate, but I still
haven't received an answer to my primary question: "Would a
professional gardening or landscaping service be a smart investment in
dealing with horsetails?"

My wife was told by one service that if they came out in the spring,
and sprayed $500 worth of chemicals, it would eliminate (or severely
improve) or horsetail problem. I tend to believe that we could do the
same thing for a lot less by whacking the weeds and using glyco or
casoron ourselves.

Anybody have any advice on which way to go? Is it worth the money to
hire experts or is this just throwing a lot of money away on the
inevitable?