The best photo browsing program that I found after almost 3 months of searching and trying different things is "Xee":
http://www.macupdate.com/info.php/id/19978
Having used IrfanView on Windows and being an advanced hobby photographer myself, I really hated the way iPhoto forces me to work. First of all, when I browse my pictures, I dont want to start a program whatsoever that can do fancy things with pictures. I want a preview-like thing, but Apple preview cant browse a whole folder by pressing the right arrow key (a completely incomprehensible limitation).
I tried PhoenixSlides, ViewIt, GraphicConverter and a couple of others, but Xee is almost what I want. However it's still missing a good EXIF data integration. I'd like to be able to display selected EXIF tags directly in the photo window, the way it can be done in IrfanView. I dont need an ugly big extra window giving me ALL 200 EXIF tags, I just want aperture, ISO and shutter speed, and displayed right on my image.
I'm really surprised that it's so hard to get a decent, lightweight, configurable photo browser for advanced photographers on the Mac. I thought there are so many graphics and photo professionals working on macs...