The thing that makes them unique is that they sprung out of a Heckler & Koch that must have been a paradise for nutty constructors. These pistols got designed in the late 60s. Rewind the tape and think about what handguns they carried in those days. The P9S must have been beamed down from the mothership.
Even today, any constructor that sat down at the engineer teams monday coffee and proposed a new model, made up of a stamped steel frame with lots of welds, draped in plastic, carrying a cold forged polygonal barrel with an MP5 roller action in a stamped and welded-to-perfection steel slide must be regarded as... well.. mad! Especially when he adds requirement for a decocker, a cocker, enclosed hammer and accuracy that rivals that of the SIG P210. And oh yeah, we're going to make a 5,5" barrelweight sportmodel too!
Ten years later the same guys came up with the squeeze-cocking, gas-retarded compact P7.
My picture of german engineers in the 60s and 70s (maybe even today) are guys that dress like university professors with cardigans and pipes, not loons that thinks out of the box. Obviously the design group of H&K was different. I doubt that we will ever see that type of wild design again, primarily due to costs. Today, everything is about cost effective production and big sales.
As a pistol, the P9S is very ergonomic, is extremly well made and has built in accuracy that very few pistols of today comes close to. The only ones I can think of (as the P210 is out of production) are the SIG X5/6 and perhaps the Pardini GT45/GT9. Keep in mind that the P9S is a service production gun, not some fancy custom built 1911. It has been in service with the Navy SEALS and dragged around the world in mud, dust and saltwater.