@roberjerman, Music Reference's Roger Modjeski has thoroughly investigated the Futterman, and thinks highly of it. He has confirmed the amp's low output impedance, and other superior operating characteristics. The only negative I have heard about the design is in regard to reliability, due to what I don't know.
Harvey Rosensberg (and his engineer George Kaye) made some changes to the amp when he started building them under his New York Audio Labs company name, having bought the rights to the design from Mr. Futterman shortly before the latter's passing. They too are known to be failure prone. Ralph Karsten's Atma-Sphere OTL's, on the other hand, are known to be very reliable, though they suffer from somewhat high output impedance (relative to the Futterman).
@shkong78, yes, OTL's are known for their transparency, but many planar loudspeakers have a nominal impedance lower than OTL's like to see. Magneplanars, for instance, have an impedance of 4 ohms and lower, a tough load for an OTL. Atma-Sphere's Ralph Karsten recommends using an Anti-Cable Autoformer with his amps when partnered with low-impedance speakers such as Maggies. Most contemporary ESL's also are low-impedance designs, at least at certain frequencies.