Copley is a true nightmare at street level, but a very successful mixed-use indoor street in its own right. Along with the Prudential Center they serve a useful purpose in the city. Of course I do often think, as I walk through these spaces, about how they are not truly "public" - subtle and not-so-subtle cues are meant to define the space as exclusionary and curtail any public gathering that could be contrary to the capitalist interests of the shopping plaza. But then again, to have an indoor shopping plaza that is mixed-use and T-accessible is an overall benefit to the retail mix in the city.
I have a bit more of a problem with Cambridgeside, although I think its future success as a more urban destination rather than a suburban interloper depends on the transformation of McGrath and the surrounding NorthPoint development.