Ranked in these QuestionsQuestion Ranking
Pro Some editing tools included (cropping, color correcting, etc...)
Pro Recognizes virtually any image format
Pro Ultra fast
If you press and hold the right-arrow button (for displaying the next image), you can watch images pass by as if they are a video, it's that fast. No stuttering, it loads images as fast as your hard drive can keep up.
Pro Lots of plugins available
Plugins can be found here.
Pro Batch processing
Even exif information can be batch processed.
Pro You can just scroll with your mouse to explore images
Pro Can use 8bf (Photoshop) filters
Con Preferences are hard to find
Con GIF viewing is buggy
GIF viewing has longstanding problems like flickering, incorrect framerate or artifacts, and it is very unlikely that it will be ever fixed.
Con Windows only
Con UI is far from ideal
Sometimes the interface is confusing.
Con No transparency support
The alpha is converted to the selected background color upon opening a picture. When saving the picture again, you can select a color which is converted to alpha again.