This post was really helpful. I spent a couple days with two and three finger swiping. Now am using two finger only. Good to know about the option key plus four finger in finder.
Here's my summary of the options (as I mention below, you basically need to choose what to use three fingers for):
Swipe between desktops with three or four fingers: nice to have the redundancy, but then you can't use the three finger gesture to go back and forth or move around windows
Swipe between pages with three fingers only: you lose the awesome animations in Safari and XCode, but it should work pretty much everywhere
Swipe between pages with two fingers only: you have to use the option-four-finger trick to go back in finder, otherwise you can use three fingers to move windows (also works to select text I've noticed), and you can use the two finger gesture in XCode, Safari and Chrome. Chrome has an arrow dragging across the page animation that I like less than Safari's full page slide
Basically the three finger gesture is your big decision. You can use it for moving windows, switching desktops, or scrolling.
It's nice to not have to worry about putting all four fingers on the trackpad to switch virtual desktops, which is probably why they left it that way by default. Also because the virtual desktop animation is one of the smoothest in the OS and they wanted to make sure it got highlighted.
Moving windows with three fingers is nice and works well, but I run out of trackpad well before I get to my second monitor. I also kind of wish I could move a background window without it taking focus (thought there was a keyboard shortcut to do so, like holding option). You'd think it's not that different to just hold down the clicker while you're moving the window (since you have to be moused over the top bar), but it actually is a different experience. The three finger shortcut is a much nicer way to move a window. If I had a smaller desktop or commonly used lots of smaller windows (maybe like photoshop, or something like omnifocus that has lots of inspectors)... if that were the case I might be more likely to leave this turned on.
It's nice to be able to use the three finger gesture in finder, but since most of the apps I use support the two finger gesture (which seems to be Apple's preferred forward/backward navigation gesture going forward), I don't know if I want to "use up" the gesture on something that two fingers generally does nicer.
All told I'm loving the gestures in Lion--Apple really changed my experience of using a computer day to day as a Software Engineer. I think maybe now that I know about the option-four-finger trick I'll go back to three fingers switching between desktops. I miss being able to do so without making sure I have four fingers on, and I think I'll give some more time to using things as Apple originally designed them.
[Edit: The option "Swipe left and right with three fingers" for "Swipe between full-screen apps"----it actually allows three or four fingers. I think I like this best.]