Well, I got my Magic Mouse a few days ago, and I've been using it for my everyday tasks for a few days (and I'm still using it), and here are my impressions. I love it!
Once you've installed the Wireless Mouse Update, you'll want to go to the Mouse Preference Pane in System Preferences and click the new "Set Up Bluetooth Mouse..." button. You no longer have to use the Bluetooth Setup Assistant to pair, which makes it a little simpler. It will then search for your Magic Mouse, and make sure it's turned on, which is done by a switch on the bottom of the Magic Mouse. Once it's paired, you'll be all ready to use your Magic Mouse.
Clicking still feels mostly the same as it did on a Mighty Mouse. If you've used a Mighty Mouse before, you'll already be familiar in how clicking works. However, there's no side button or scroll ball that can be clicked. You can configure different actions for the two "buttons" in System Preferences. Scrolling works well, and again, if you've been using a Mighty Mouse or a mouse with a scroll wheel, there's little to get used to. I just never liked that pesky ball in the middle of the Mighty Mouse. Also, swiping works just like it does on a Multi-Touch trackpad, which can be used for many different things in your favorite applications.
Overall, I am happy with the Magic Mouse. It is beautifully designed and quite functional. What will Apple think of next?