This calculators real value lays in the inclusion of the source code and a very detail, step by step tutorial on using project builder to create the calculator. This makes it a great example for people starting to learn Cocoa.
The calculator itself is flawed in that it doesn't deal with order of operations properly (ie 2+3*6 results in 30 -- (2+3)*6 and not 20 -- 2+(3*6) (as calculators should)
But given that there is source code here, it shouldn't be problematic to add a stack class to the code to deal with order of ops properly.