I work on a product that has millions of lines of code and has been in existence for over 15 years. It uses Interface Builder and has a very complex interface.

I work on a product that has millions of lines of code and has been in existence for over 15 years. It uses Interface Builder and has a very complex interface. IB works just fine. It has quirks like any other tool you'll use, but dealing with it is no worse than dealing with the changes to compilers, graphics drivers, OSes, etc.