And as far as usability, command-space from any focus is a pretty big step forward from OS9, OSX before spotlight and Windows search.

The Finder's spotlight interface, while it doesn't include OR, is hardly difficult, very capable and better than most.

I read a post somewhere (on here I think) about doing boolean (not/or) searched in spotlight. Never tried it so I don't know if it works in Snow Leopard, but I believe the pipe (|) was an "or".

Why is "command-space" better than "command-F"?

You speak like someone who's never even used OS 9, (or for that matter, Alta Vista).

Convenience. As edified says, you can do cmd-space from anywhere. For cmd-F you have to be in the Finder. ( Plus, arguably my favorite feature added in Leopard is the calculater in Spotlight! =) )