Picked up my second Rokkor lens the other day and I'm loving it. It's an excellent copy and has no haze, fungus, purple fringing, or other flaws that I can see.
Buying lenses used you have to look for these things, as well as testing the aperture ring since that part is manual. Look through the back of the barrel and check out the inside to see if you can spot anything on the glass. Older glass may have some dust but if it wasn't kept in a cool, dry place for long then it will show signs of fungal growth, haze, and worse inside the barrel.
I was very excited to get this from the local camera shop for $25. Some of these go for $50-$100 on ebay and other resale sites. The only thing needed is a MD-NEX mount adapter that you can get from Amazon.com for cheap. Mine is the FOTGA MD-NEX as you can see from the picture.
Some of the images I've taken are seen below in the gallery. These are at f 1.7 and ISO 125 straight out of the a6000.
I'm still working on nailing the manual focus. At 1.7 you don't have much room to miss so a couple of these are not super sharp but it's my fault. (You can really appreciate optical stabilization in the newer OSS lenses!) However, you can get the idea of the depth of field you can bring in with this lens. Everything falls off beautifully in the background. There is a nice circular bokeh as well. I'm using these on a crop sensor and I get no vignetting even with the adapter which is a huge plus!
These lenses are a very cheap way to get into using your Sony camera as a walk about without having to spend $700-$1000 on lenses like the 24mm E mount or the 55 1.8. Yes they are sharp, yes they are autofocus, and yes they are 50x the price of this lens :)
I am now just awaiting the a7rII so I can get a full frame and really see how much depth of field I can produce.