What kind of post is this anyways, the more free software the better =P
Anyways, to me they fulfill different roles. I use picasa to get pix off the camera/card reader and then organize them and see what I want to upload to my gallery site. Irfan I use for its simple resizing options and more importantly for its easy batch processing.
Picasa is really great for finding all the pictures on your comp and sifting through them. I dont use it often, but I had a bunch of pictures from over the years backed up to various places and one use of picasa got me caught up with all that i was missing. Using it for my camera interface just keeps it better organized for me.