Beware, Jason! I got these on DVD several years back, and I was all tickled to watch them, since I hadn't seen any episodes since the 70s. But I must warn you: seasons 2 and 3 do not hold up.
Even in college I remembered that 6-year-old me knew the acting was over-the-top awful. But the stop-motion was pretty dang awesome, and look at who wrote the scripts that first season: Ben Bova! Dorothy Fontana! Norman Spinrad! Larry Niven! Walter Koenig! David Gerrold! Even Theodore Sturgeon stuck his toe in in Season 2. Money can't buy writing talent like that anymore, especially for a Saturday morning kids' show.
But Gerrold and most of the real SF talent left after Season 1. Sturgeon was a lonely exception. The writing got painfully awful after the first season. There's a bright spot: The first season finale works perfectly well as a series finale, so you don't even have to feel particularly obligated to watch the other two seasons.
But as a kid, I was too in love with Torchy, the fire-breathing Dimetrodon, to turn off the TV when a Season Three episode came on. Torchy was cool.
ETA: having trouble with the spoiler tag. Oh, well.