People who bitch that hard about Lost in Space are taking it all too seriously. LiS was a good counterpoint to Star Trek's serious take on the genre. And in the 70's every SF TV series was compared unfavorably to Star Trek. Until Star Wars changed the game, Trek had to be every SF TV producer's millstone.
No matter my opinion and even after all these years I still think the Jupiter II is very cool. It seems such a pity they designed a cool ship and then after the pilot (worked into the first few episodes) we never saw it fly again until the third season, which by then the show was in colour and so ridiculous it was practically unwatchable.
Fortunately, the Jupiter II did fly in the early episodes of the second season beginning with the first episode "Blast Off Into Space". However, it was grounded after four space bound episodes for the remaining second season twenty-six episodes.