Excellent guest starring role by Robby!
Great story and while it's not surprising you feel Will's pain at what he's inadvertently done (which is carried more by Mumy's acting than the hardly shabby script), you actually feel bad for the Robot too. When he brings vegetables he picked unprompted back to the family to try to compete with the vastly superior Robby, it really kind of gets you, especially since the Robinsons don't seem to even realize why he did that.
From a production standpoint, having the Robby worked well because it's a very fine prop, having been built on a major film's budget vs the B-9, built on a tv budget. But this is expertly used as a plot point-- Robby IS a fancy schmancy 'robotoid' while the B-9 is a simple utilitarian design and looks like it. In effect (to me at least) this actually makes the B-9 seem more real.