just picked it up today from SLR revo. i was using the mark 1 for several years and i must say that all the reviews you read on the web are indeed true.
this lens is tack sharp right out of the box and for mark 1 users, the difference is immediately apparent. you'll notice that your images have significantly more contrast and edge-to-edge sharpness wide open.
pros
1) it's 150g lighter and 10mm shorter - the mark 1 is heavy so any reduction in weight is definitely a huge plus
2) needless to say, razor sharp at f2.8, even better when stopped down (no peers in terms of sharpness, according to DxOMark)
3) uses a little more plastic in the construction but the the build is still impressive
cons
1) probably the price? but the significant increase in the IQ justifies the premium you pay
2) the hood - it's dramatically smaller, i prefer the one that came with the mark 1