Spray on systems are becoming much more widely accepted now a days.
They vary in the materials used and the length of the warranty. I know
they can come in much cheaper compared to the cost of total roof
replacement and usually cheaper than new roof overlays. Obviously you
must do your homework and decide which is best for your situation. I
would compare total lifetime costs and think about any added benefits
such as reduced energy usage through reduced cooling costs or
renewable warranties and reduced maintenance costs. I am not going to
recommend any one product since my opinion may be biased since I do
work for www.icc-astec.com , Insulating Coatings Corporation.
I think if you do the research and read the articles from the major
roofing magazines and then get references and literature and then do a
full cost comparison, you will see that spray on systems can be a very
effective and economical choice.