I have had a couple customers recently ask about FloorMuffler as a underlayment option, however, i have not worked with that particular brand yet.
I have used and recommended a similair synthetic solution called The Sound 6 Barrier Acoustical Underlayment which has performed with awesome results including in condo situations. (even high rise developments in New York City (NYC) which have some of the most rigid standards.)
For folks who wish a more green underlayment i typically recommend the 6mm Cork Tiles.
Technically the synthetic is a bit more sound resistant for downward transmission, but I think the comfort of cork along with the earth friendly attributes can be a big reason to consider that.
