First the good points; the episodes look and sound great and there are a couple of nice extras including a commentary on the pilot episode from all the remaining cast members.
I loved all the Irwin Allen shows as a kid and have enjoyed them on their DVD releases, all that is except Land of the Giants; the series is repetitive and the stories themselves range from OK to infuriating to downright dull. The giant FX are still impressive as is the wonderful Spindrift space transport craft, but oh my those plots!
An example - in one story the crew encounter time travelling aliens who have a device (well, a bit of cardboard with a radio dial) that will allow the crew to go anywhere in time and space. Instead of using it to simply gather the crew and transport to Earth, the plot involves two of he crew travelling back to the day they left Earth to try to alter history by refusing to get on the ship. Never mind that their previous selves would still get on! Never mind that they spend the whole episode hiding behind doors rather than doing anything! Very silly!
As a nostalgia kick I recommend watching a few episodes, but the first season is far superior. Sitting down to watch all the episodes in order is frankly a bit of a slog.