Leroy Dickson wrote:
How  many of you use them a lot? Do you know most of them?
Does it speed up your work significantly?

Personally, I use very few of them.  I'm a clicker Can't for the life of me remember many of them.

All the time; they're HUGE time savers.

D for default color, X to swap foreground/background.
Numbers for opacity of brush
Brackets to resize brushes
S/J/B for cloning, healing, or a regular brush
Function keys for favorite actions
et-very-cetera

I typically recommend a frequent PS user learn ONE keyboard shortcut every 3-4 days; the one function they accessed the most with a mouse. After a while, they'll have learned most of what they need, and can stop. (I figure I use 20% of the built-in shortcuts at the very most.)