Well - picked one up from PC World this lunch time (Impulse buy.... 60 quid )
First impressions - shorter and fatter than the mx700 (my previous mouse) - shape is much more like the MS Explorer v1. Much bigger thumb grip - extends about a centimeter under your thumb.
Cruise up/down is now a rocker, as are the back/forward buttons on the side. Task switcher is now in between front/back on the side. All buttons feel significantly larger. and more durable.
Weight is less than a mx700 with batteries - but more than a wired mouse. Took about an hour to charge.
Laser is invisible - so don't bother looking... (I did, more than once, but I'm blind enough already ) - it's up in the infrared.
Goes into sleep after about 10 seconds inactivity - wakes up immediately - like it's not even asleep - about the only way you can tell is that the battery lights go off. Dunno about battery life yet
No perceptable lag - at all - seems even better than the mx700.
Accuracy is better than any mouse I've ever used - in fact - I'd put it up there with a CAD tablet - it's that good. I can put the mouse speed up to transfersing a 1600 wide screen in 1/2 inch - and still track down to individual pixels.
To sum up, at this stage the mouse appears to be fantastic - though I'm not sure yet about the Li Ion battery - I was able to keep two sets of spares for my mx700 - not going to be able to do that with this one.