I guess my comment is yet another "me-too". I too am faced with the problem of the excessive sensitivity of the magic mouse - but also of the trackpad.
Both work fine with regular interactions: scrolling in "regular" documents: text, preview, web pages. But outside those uses, the scrolling becomes erratic. Most complains are with web mapping applications: google maps, but also bing maps, and pretty much all of them. I first noticed this when using our own map applications, similar to Google maps.I first blamed it on our developers - until I noticed the same issue with Google Maps.
But the issue also happens with tools such as MS Powerpoint or MS Word and even Excel. In all case, a small touch on the mouse triggers a massive scroll. That happens to me all the time with Powerpoint and Word: I am typing some text, then I want to move the mouse to a place some lines above or below - and suddenly the document scrolls wildly to some place far away in the document. I then waste time moving back to the slide I was working on. Not good for my productivity. On Excel, scrolling precisely to the line you want to see or modify is very hard. Scrolling just one line up or down is just not possible at all.
Interestingly, Apple-made tools seem not to suffer from that: Preview scrolls smoothly and precisely. So do iTunes, App Store, Messages or Mail. I have not tried Pages, Numbers or Keynote, but I am curious about the feedback. In those tools a small movement with your finger on the mouse translates to a short scroll. A longer movement translates to a longer scroll. But in MS Word, even a short movement translates to a scroll of half a page or more.
This demonstrates that the issue is NOT with the magic mouse or trackpad proper, but rather with the applications that handle mouse/trackpad scrolling events. When I move my finger slowly and just a tiny little bit on the mouse, the form I am typing this in (using Chrome) scrolls just a tiny little bit at the same speed - clearly proportional to the movement of my finger. If I do the same tiny movement in a Google or Bing maps application, it zooms in or out wildly - not just one zoom level as I would expect.
Of course, the major difference between the magic mouse (or trackpad) compared to a regular mouse wheel is the absence of any feedback: the mouse wheel clicks when you scroll. Typically one "click" means zooming in exactly one level in map applications (or advance one slide in Powerpoint). The magic mouse has nothing like that.
The bottom line: the magic mouse (or the trackpad) is useless when scrolling in most non-Apple applications: Web Maps (not just Google, also Bing, Yahoo, Here, ... ) as well as productivity tools (MS Office). I can't imagine that this is all part of a devious plan of Apple to get us to use their own mapping app (yechh!) or their own productivity suite :-)
But does Apple listen to our complaints ? Does it do anything about the bug reports we file - besides closing them as "not a bug" ? No it does not. We are just customers and we are obviously wrong: too stupid to use whatever technology Apple makes (but not so stupid as not to buy it). I know that ******** and moaning in these forums makes no difference, but it is the only place I can express my dissatisfaction.
I am now back to using a "regular" mouse with a real wheel. Works fine in all environments. Thanks, Magic Mouse and good bye.