While reedless accordions have been around for a very long time, I have to say that the Rolands are the only ones, at least to my ears, that come the closest to actual acoustic accordion sounds. I have used some accordion sounds with the Ketron and the BK but with those units, something seems to be missing in the sound. The kind of expression you can only get with an instrument designed to be used with bellows, the multitude of acoustic sounds available. Unlike other synthesized units that give some accordion sounds, the Roland was designed from the get go to closely mimic accordion sounds and it shows. And unlike module units like the BK and Ketron that try to interpret the bellows pressure and generally give a kind of chunky, inconsistant bellows response, the Rolands do a pretty fair job of working with bellows expression.
Replace acoustics? Never, but not a bad imitator of them. As Jim D said, they can actually sound pretty sweet.