Originally posted by jolepp A raynox 250 is going on a local auction site, currently for 6 € ($8). What (if anything) would this be good for?
It is good for taking superb macro photos.
The Raynox 250 is almost twice as strong as the 150 (actually 2/3 stronger), so is suitable for shorter lenses (like shorter than 100mm... it'll achieve a magnification of about 1:1 with a typical 85mm lens). Its drawback is that its working distance is only about 125 mm (5") compared to the 150's 208mm (8").
Your built-in auto-flash will work with a Raynox 150 on a 55-300mm lens. I doubt the flash can directly illuminate the subject using a Raynox 250 on a 100mm lens, but just reflecting it with a piece of paper isn't all that tough.
The shorter working distance for the 250 (& lighting problems) may interfere with wildlife shots.
While I think the 150 is more useful if you've got a good long lens, the 250 is a likely valuable addition to your kit.
Dave