PSB files have been around for quite some time. However, a lot of people are unaware of their existence. PSB files are photoshop files that can be larger than 2GB. This addresses the short coming of PSD files, and their 2GB limit.
When I have my large format film scanned, the result is a very large file. I often scan 4×5 film at 1.2 GB or larger. The moment I add a second layer, the file size will be larger than 2GB. Before I knew about PSB files, I had a major dilemma. I was forced to either leave that file open on my computer for weeks on end until I was done with the edit, or flatten it and save the file as a Tiff.
I use a 5 year old Mac G5 that is loaded with RAM. Although the new Mac Pros are at least 10x the speed, I have not found that to be a limitation while working on these massive PSB files. Although the files are large, the responsiveness of Photoshop is not hindered.
It does however take a very long time for me to save and open the files. It will often take 20 minutes to save the file. This is something that will be greatly improved with a new computer.
For digital shooters, PSB files will be very important if you do multi-shot stitching. Once you cross that 2GB limit, PSB files will be mandatory to save the image with layers intact. If you choose to use this format, it is listed under “Large Document Format” on the Save As interface.