Lion, but so far I like what I've seen and have adjusted to most of the changes easily since most of them are logical and intuitive.
The gestures for scrolling through lists and Web pages, for example, make more sense a it is like pushing a piece of paper? Want it to go "up"? Move your finger up on the scroll pad/Magic Mouse. From that you can probably figure out "down." That works in the panes of Second Life, too, like for scrolling through your inventory. Zooming the view is another mattter.
Previous to Lion, if I moved my finger on the tracking surface of my mouse (rolling the wheel away from you for Windows users), Second Life's camera would zoom in. Moving my finger toward me, as if scratching, the Second Life camera would zoom back. Not so in Lion. It is just the opposite. That will take some major getting used to before it becomes natural. Using Ctrl-Option-Click to orbit/truck the "camera" still works.
Generally, though, my computer seems snappier and even Second Life seems crisper in appearance and in function. Now if only I could get Netflix to work again ...