Note that you can bring your mac's gamma up to 2.2 right now without upgrading: Open System Preferences, Go to "Displays", then the "Color" tab, then click "Calibrate". This will give you the option of setting your gamma to 2.2.
I'm trying it out myself right now. Seems very different, not sure which gamma I like more.
On pre-Snow-Leopard systems, all of the UI is going to look dark and contrasty, contra system and application designers’ intent. If you like it, go for it, but I think it looks terrible. With Snow Leopard changing the default, developers will of course design their apps to fit γ = 2.2, and will lighten everything up, so that it looks right.
I have always used and been a fan of 2.2. It makes sense to use what most others use, imo. When you choose colours for a site you know that most people will see similar colours.
With a gamma of 1.8 I found that on other machines using 2.2, light colours I chose tended to look white and dark colours looked black.
I'd imagine that Snow Leopard makes various tweaks to the source assets to look better at 2.2 gamma. Changing your preferences will have the desired effect on web pages and such, but not necessarily on native controls and system UI.
I'm trying it out myself right now. Seems very different, not sure which gamma I like more.