It's vastly better. Auto focus is much faster which was the main problem with the old body. I have not had a chance to use it for a full day of studio shooting yet because everyone is on holiday. Will be able to get a much better idea towards the end of January when everyone's routine goes back to normal.
I am not getting a new back. The IQ160 is perfectly good and it would not be worth my while to upgrade just for a few extra features. The upgrade would set me back another $10,000 at least and would not get me any better image quality.