Honestly, I had mixed feelings when my new iMac27 came with that tiny wireless keyboard and the new wireless Magic Mouse. But WOW! The Magic Mouse rocks. It's awesome, terrific, wonderful and the best ever. After a week with the new iMac I cannot imagine ever using another mouse again.
What makes the Magic Mouse so great? It feels like you're using an iPhone! That's because the top of the Magic Mouse is really a Multi-Touch surface with gesture support. So instead of driving a scroll wheel or that funky microball in earlier Apple mice, you scroll exactly like you do on an iPhone. There's the same wonderful precision and, just like on the iPhone, you can scroll with "momentum," i.e. whip up and down pages (or any other direction) and then stop on a dime.