Why are most comments referring to having used this in the past tense? I was under the impression that it was still the best image viewer in town, on Windows at least
The built-in viewer in windows is fine. I can't really think of a feature that it doesn't have that I need. Could you say why irfanview gets your vote?
It's also a paint tool (edit->show paint dialog), and does tricks like swapping channels or repeating the image as a grid of tiles, which are handy when programming something involving raster graphics or textures.