The Commodore 610 is also known as B128 in the USA. Basically it was meant to replace the PET series. The hardware consists of a bank switching 6509 CPU, 128K RAM, IEEE interface, a 6545 video generator and a 6581 SID chip! Thus it is black'n'white, but in Germany there were some 3rd party expansions to get a colour picture I've heard. Other than the SID chip, it doesn't have much in common with the C64. Software wise, it is barely compatible with the PETs, which generated a lot of work for software houses to rewrite their business software to run on the CBM-II series, as they also are called.
The 610 is known as a low-profile Commodore, as it doesn't have a monitor. There is a high-profile version called 710 (or 720 for the one with 256K RAM). I have come across a half dozen 610's and a few 710's from the PET place, and sold many of those. For that reason, I don't find them particularly "ultra-rare", but if you never have seen one before, they appear so.
Due to the shortage of software, I think they're mainly collectable vs useable. On the other hand at the Commodore show in December, in the one where Jack Tramiel, Stephen Wozniak et.al. were having a panel discussion, at the end there was a gentleman from the audience praising the B128 for being a truly superb computer of its day, outclassing both the Apple II and IBM PC. He asked Jack why Commodore didn't put more efforts in marketing this wonder computer...