I popped into the Apple Store recently to check out the mouse; and it lives up to expectations, if not to dreams. It's physically very similar to the pill-shaped Apple Pro Mouse, except that it has a small ball-nipple in the middle. If you're used to a Pro Mouse, it's in the place where you'd want it to be for general use; however, it will no doubt be in an awkward place for non-Pro Mouse users. Similarly, if you're used to it, the squeeze buttons are in exactly the right place, since that's where you'd pick the mouse up if you wanted to drag an item after having run out of desk estate. (It's not clear what would happen if you dragged the Mighty Mouse; it might activate the so-called 'squeeze buttons'; or these might by disabled during a drag operation.)
The scroll ball will be great for people who only occasionally need to scroll left and right, as well as up and down. It's not a replacement for a scroll ball if you use it a lot, but it's good enough for every now and again, which is what I expect it will be used for. It's certainly better than the 5-way scroll wheel on a Microsoft mouse. Unfortunately, it's not clear how easily the ball can be cleaned if required; so expect some of the first-generation mice to pick up some cleaning dirt.
Many people have already lamented about the lack of bluetooth support. I expect that bluetooth will come eventually, but not any time soon. (My money is on the Paris Expo or one of the similar ones 2006Q1. Expect a tag line such as "The Mighty Mouse: Now Even Mightier" when they launch it.) It's really well designed for people who have a desktop Mac, as the (non-wireless) keyboards have a USB dongle that will happily power the mouse itself.
Of course, the other Big Thing is that the Mighty Mouse can deal with either a single-button or dual-button configuration. This is great for people who might accidentally use one not knowing about right-clickcs (a small but demographically important group of poeple), whilst at the same time allowing another power user to use the right-click options. I could install one on my kitchen computer, safe in the knowledge that when guests come to browse the internet they'll be able to use it without fear.
The design is very clever; the top of the mouse is solid plastic (apart from the nipple sticking out) so there's no physical sensation of right or left buttons. Instead, depending on where the fingers are resting at the time of the click, it is either interpreted as a 'Primary' or 'Secondary' button click (configurable for either to appease SouthPaws). By default, both sides are interpreted as Primary, giving the impression that it's a single button mouse. It's somewhat unfortunate that in order to register a Secondary click, any fingers resting on the Primary area need to be lifted off first. You can rest fingers on both areas for the Primary; optimising for the common case seems worthwhile. It's mostly nice to have a second button, but it's not used most of the time, so this is a reasonable trade-off, but for people who do a lot of right-clicks in Photoshop or similar applications may find it overly annoying and/or develop a kind of reverse RSI by lifting instead of depressing the finger all the time.
Is this a Mighty Mouse? Well, it's great; but perhaps not Mighty. Although the squeeze buttons give access to (configurable) Expose or Dashboard, I'm not sure whether that many people will find the squeeze buttons that useful. The scroll ball is pretty good, but definitely judders when scrolling along a diagonal; it's probably not something you'd want to use for detailed artwork; on the other hand, scrolling pages in Safari -- the main target -- works well.
If you're holding out for a bluetooth version, expect to wait some time. Whilst it may appear good for some advanced users (and techno geeks such as myself), expect the revision 2 to have fixed some of the problems with the diagonal scrolling. Further down the line -- maybe in the middle of the great PPC depression -- expect to see this mouse packaged as the default, in the same way that the Pro Mouse is now packaged with each new PowerBook. And if you really want a good wireless mouse that can be used to scroll in any direction, perhaps you should consider the GyroMouse instead.