The Retinas have quite a following, and can take very good pictures as they have pretty decent Schneider or Rodenstock lenses. The usual f/2 lens is pretty fast too, for the times they were made.
Retina II's and III's have coupled rangefinders. They are worth $50.00 - 100 USD depending on condition and the exact model, though sometimes you can find them for a good bit less.
I have found that they don't take non-use too well, so I'd recommend a CLA before trying to use it seriously. The bad news is that the CLA will likely cost more than the camera's low resale value, but it will be a good an reliable user afterward, and will add to the camera's value
An excellent source of information about them is Chris Sherlock's site at http://homepages.ihug.co.nz/~Srawhiti/index.html
I recently had one of my II's CLA'd at Zack's camera repair in Providence RI. They did a fantastic job.