I had good luck with my first three family that had a home warranty. Had a massive hot water heating leak under a tub that they fixed for the $35 call out fee. They also did a $450 heating system repair for the call out fee.
BUT - I had a three family that had separate heating and hot water for each unit. When another unit had a heating problem, they refused to repair because I had already claimed a heating repair. In the end I got rid of the policy because I was being charged for three family coverage but didn't feel I was getting 3 family coverage. I also read horror stories about the company I was with, and eventually I was part of a class action lawsuit against them.
I eventually decided that the money I was spending on the home warranty system would be better saved and put towards actually upgrading the old systems. I think my policy was $1,200/yr. Yes, I've spent more on replacing the furnaces, etc. than I was saving but I haven't had to pay for service calls either.
If you go the home warranty route, make sure that you read all the really fine print. Most policies won't *actually* replace the system, and the total amount paid for a major repair is capped. Based on the repairs that you've already had, do a cost benefit and see if you would be ahead or not.
In my experience, you most likely wouldn't be ahead if you went the home warranty route, but it might be a short term solution to get you through a year or two more in the black so that you can upgrade the systems.