Viewed as science fiction -- a realistic prediction of future events based on known scientific principles -- Space: 1999 was a dismal failure. While the basic setting of Moonbase Alpha, and many of the spacecraft depicted, were all of sound mind and body, the stories themselves blatantly ignored everything known about celestial mechanics and allowed the Moon to wander wherever the script writers needed them to go.
Now, take a step back. Don't view it as science fiction. View it as space fantasy. Forget reality, or even verisimilitude for a moment.
Viewed in this light...well, to be dreadfully honest, the show still doesn't fare extremely well, but it doesn't fare nearly so badly, either. It places its regular characters into situations that stress them, horrify them, drive them to the breaking point, to see what they do to retain their humanity. The first series, in particular, spends a lot of time dwelling on what it means to be human when all the usual trappings of humanity have been stripped away; when everything around you is not merely alien, but contradicts everything you thought you knew.
Could they have done it better? Probably. But if you set aside your preconceptions of what "science fiction" has to be, you'll find there's actually some good storytelling going on under all that glitz. Yes, even in the second series.