A lot of creative types have been holding off buying a new laptop in anticipation of this, and its new specs are pretty much in line with expectations.
You now get Intel's Haswell chips, and the same company's Iris graphics. Battery life has been improved, and it's flash storage all the way. Better still, prices have come down - the 13-inch model starts at £1099/$1299 and the cheapest 15-incher costs £1699/$1999.
Should I buy it?
If you want the best Mac laptop for music making, yes. The only downside is that, with the move to lower capacity flash storage, you may have to carry a portable hard drive around with you as well (for storage of sample libraries etc), which arguably negates the benefits of the laptop itself being lighter.