I think ACDSEE is by far the best photo browsing application. iPHOTO really lacks that capability, as it forces you to store all of your folders/pictures into a database. To make things worse, it copies the photo instead of the location into the database, which means you end up having a huge repository on your local hard drive.
I also tried Aperture, which is a little better but it also does the same thing. By far I think JustLooking is a great app for quickly viewing a picture, and the CocoviewX.
While I think it is a good idea to simplify applications for most users, it is a pain in the butt for the power users. I like to know the location of my files, as I can keep track of where each file is. When it is controlled by an application, it is easy to accidentally delete things.
Again, I got to hand it to Microsoft for creating a better file management application than Apple (Explorer kicks Finders ***). However, I will also admit with some tweaking, Finder is bearable.