This lens is a worthy contender to the legendary 1.2. Whilst the 1.2 has its own special place, this lens with its 4 step IS, finally a useable AF speed and better sharpness makes it a winner.
The build quality is superb too. One nuance with this lens (in case you get worried) is that the IS activation and deactivation is quite loud, one of the loudest I have heard. Everyone I know with this lenses has the same, so it is not an issue!