For simple Picture viewing there's still nothing better than ACDSee classic, in my opinion. Excellent keyboard command for zooming in/out and file manipulations, very fast image decoding, full screen mode that works just the way you want it (you can enter/exit from it or switch to a folder thumbnail mode with a single mouse click), quick image conversion, lossless JPEG rotation, and just general user friendliness and compact form.
Picassa 2 is good for image catalogization, but compared to ACDsee classic, it's bloated and inefficient for image viewing.
The good thing about Irfanview is that it has scaling algorithms that are probably better than anything even Photoshop can offer, especially if you have to upscale the image into higher resolution.