As a UI designer, there are few things more soul-destroying than having to ask a coder every time you wanted to move something by a pixel or replaced with something that (to them) looks identical.

My gripe with IB is that it is beginning to look long-in-the-tooth: it lacks many of the modern layout tools you might expect: zoom, show/hide layers, lock, align-just-these-together. IB could do with taking features from a vector art package (without becoming too-much like Expression Blend).