If you download the OS X Lion from the app store, you wil see in the app bundle when you select "show package contents" in finder you can navigate to one of the subfolders and you'll find a 3.5GiB dmg file. I burned that file to a DVD and formatted a 1TiB external USB disk drive as an HFS journaled fs. The DVD was bootable and I installed Lion to the USB drive.
The point I'm getting to here is that you can purchase the "upgrade", burn it to a DVD and then easily install it to an external drive if you want to test it out before upgrading.
I wasn't impressed with inverting the scroll direction on my mouse. I turned that off, "natural" my ass. I don't like what they have done with the scroll bars through out the UI. The space for the bars are still there only now they "hide" so you can't see where you are in a document. That's irritating. They've screwed with the "back" and "forward" function on the Magic Mouse. What was "back" now makes the whole screen slide to dashboard. Annoying. Now back and forward takes three fingers.
Expose is changed. I kind of like what they did with it though. Can still set corners to show all windows for a single app and show desktop. Keys don't auto repeat any more, holding down O for example brings up a pop-up so you can select the modifiers on that letter. Some will appreciate that, I don't. I wonder how that will affect games that use W-A-S-D ?
Ical is fugly as hell now. Mail.app has been made annoying, at least you can go back to a "classic" view. Over all, it's just business as usual. They've dorked with the UI. Seems like they've run out of things to innovate, so they do stuff like screw with the mouse wheel direction and make the scroll bars less useful. Google chrome has some issues, back and forward don't work any more and it gets trapped in "full screen". Ouch, new UI changes shouldn't break existing apps.