Dragonfly is quite nice, but we opted not to use it for a handful of reasons: ImageMagick itself is a actually a good bit slower for basic resizing/cropping/rotating than the library we're using in Photon; Photon is ~100 SLOC, Dragonfly is ~3500; Photos are the core of our business, so a generic solution is not likely to work 100% how we need it to (we have various legacy concerns that probably would've require more code to monkey patch Dragonfly than to just write it into Photon); Java is much better than Ruby for CPU and RAM bound tasks that need to execute in parallel.

All that said, Dragonfly is a great choice for most people.