The best word I can find to describe this story, I think, is "comfortable". Yes, it was long, yes it was slow paced, but Silverberg did a masterful job at pulling me into the Precambrian world and then just letting me sit there and experience a slice of these inmates' lives. And what I saw/heard/experienced was both interesting and entertaining. Not entertaining in the same sense as a Hollywood action flick or comedy, but I found myself thoroughly enjoying the setting, the characters, the many minor conflicts from food gathering to slow-onset psychosis. By definition, this was entertaining as an "agreeable occupation for the mind".
The time travel flaw I saw in the story was the understanding that time "Up Front" moves at the same pace as in the past. Well, technically it does, yes, in the sense that one day equals one day. But with time travel, when you can send something back to any time, it's silly to think that someone who just came from the future would have to come from say, 2029, because it's been 20 years since the station was set up. Yet all the inmates still think of time as having progressed from the moment they arrived at the station. I'm finding this idea harder to articulate than I thought. I hope that made sense to someone.
Anyway, although I did enjoy the story in full, with all of its tangents and side stories, I do wonder if a story of this length would get published today. With the push for authors to cut anything not relevant to the main plot, I think Silverberg would be hard pressed to get say, Asimov's or Analogue to pick it up without having to do a significant amount of trimming. That being said, I'm glad it was written back in a time when longer stories were more tolerated because there isn't any aspect of this story I would want to see cut.