For web design I would say that Photoshop 6 has introduced some good features, but overall it probably isn't worth the upgrade - mainly because of the buggy nature of this first release. Perhaps wait until a dot release fixes up some of these issues. If you use Fireworks in your work flow it will support Photoshop 5.5 files but not 6.0 (even Fireworks 4).
One of the touted features is the vector support. This is a good feature, but badly implemented. It is nowhere near as simple or extensive as Fireworks vector support.
The layers panel is good too - much more flexible and some of the live effects are a real plus, but it seems that they are very slow to apply - there is a considerable wait on my machine for the dialogue to open.
Also new was the progressive JPEG - this is a good feature, shaving precious K's of images (this allows you to specify multiple areas of a JPEG and save them at different quality, hence preserving text). FW4 will do this too.
If you are using PS for print work as well then it may be a different story entirely - there are some serious print features in PS6 that would make an upgrade much more attractive.
I think one of the best solutions is a combination of PS5.5 and Fireworks3/4 - you can do your heavy duty bitmap manipulations in PS and import the whole file into FW to add any vector data etc. But that's just me - what do others think?
gollyg
