Steve, please, for the love of god, just split the damn mouse down the middle and give it two buttons. The pseudo-button(s) on the Mighty Mouse drove me crazy. I have to admit though that I never had any real intention of holding on to the Mighty Mouse, due in part to the fact that in the past nine months I've gone through six different mice1 and because I, like a lot of other Mac zealots(?), am just compelled to try out anything new from Apple. I figured I would use it for a few days and then return it — that's exactly what I did.
Before I start complaining, let me first expound on the virtues of the mouse's only saving grace, the scroll wheel, or "nipple." I'm a big fan of the nipple (to quote Jon Stewart, "settle...") and can't wait to see it adopted in other mice. It feels perfect and the "click" sound generated by the tiny speaker inside the mouse complements it well. I was this rarr; ← close to keeping the mouse just so I didn't have to give up the nipple.
The only thing I would change about the nipple would be to make it clickable. It can currently act as a button mapped to some action, but it doesn't act independently of the rest of the mouse; when you press down on the nipple hard enough it causes the mouse itself to click (not unlike when you give the mouse a normal left-click) and the guts deduce from this movement that you're pressing down on the scroll wheel. The whole thing feels a bit awkward.
Speaking of awkward, I was never able to stop thinking about hitting the "right" mouse button and this is what ultimately came between me and the nipple. I kept telling myself to try it for another day to see if I could get used to it or maybe come up with a different way of holding it. Nothing. My annoyance with this came into strong relief when happenstance had me use my "regular" mouse again, which immediately freed me from having to think about right-clicking and I quickly realized that the Mighty Mouse just wasn't for me.
Another thing I was a little disappointed with was the length of the cord — <1m. It's perfect for me, and, I suspect, for others who use Apple monitors (because they usually have built-in USB ports), but there's just no way to use it if your tower is sitting on the floor.
Finally, the side buttons were horrible. Not only can they not act independently of each other (they must be pressed simultaneously), but they're in a horrible position and offer pretty much no tactile feedback. To be honest, I thought the buttons on my mouse were broken until I saw other people with the same complaints.