When Heinlein was at the top of his form he was just about the best out there at the time he was writing (and there really isn't any journeyman-stage Heinlein around -- one of my theories is that he was that rare thing, a natural writer with the bad luck to be better than just about anybody else in his field, which meant that he never really needed to work at improving from his own baseline level of competence.)

On the other hand, when Heinlein flubbed it, the results could be downright embarrassing -- Farnham's Freehold is pretty much the "Spock's Brain" of the Heinlein canon.