I found the Retina IIIS to be a very usable camera. The Retina Reflex S, III and IV were somewhat heavy cameras.
For the price, it's a very good camera. The original Reflex, S, III and IV are more complicated because of the mirror mechanism. Plus, the viewing system almost always needs a thorough cleaning, and is more complicated to disassemble.
Like the Retina IIIS, don't force the film advance and if you're buying one, make sure the film advance works correctly.