I don't think Silent Running completely sucks, but it's not a very good movie. The robots were the best part, but the astronaut was a bit of an idiot when he took the ship further away from the Sun and then couldn't figure out why the plants started dying. I mean, duh!
I think of Silent Running as a product of its time that doesn't hold up as well today.
"A religion old or new, that stressed the magnificence of the Universe as revealed by modern science, might be able to draw forth reserves of reverence and awe hardly tapped by the conventional faiths. Sooner or later, such a religion will emerge." -Carl Sagan