Apart from the fact that this lens is considerably shaper, the main advantage of it has over the 18-70 it that it is much faster, especially at the longer focal lengths, meaning that it will be much more effective in low light conditions.
When shooting weddings I always use two camera bodies at the same time and I envisage that this lens will stay on one of my cameras virtually all the time whilst I interchange lenses on the other. The 17-55mm is a very popular lens with professional wedding photographers as it can go wide enough for group shots but is also capable of zooming in nicely for details or candids. It is also built like a tank so I expect it to last a very long time!
I will post up some photos shortly once I have had chance to play around with the lens.
I purchased the lens from Park Cameras (http://www.parkcameras.com/) and their service was excellent. They were very helpful on the phone and the lens arrived by courier less than 24 hours after I ordered it so I would be happy to recommend them!