Unfortunately, many of the other reviewers here are on point. In Spin, we followed the lives of a small groups of characters while big ideas happened around them, spanning either decades or billions of years, depending on your perspective. In Axis, the ideas are not nearly as big, the timespan isn’t quite so vast, and the cast of characters has changed to ones I don’t find myself caring about as much.
There is a bit of an interesting follow up here to the last chapter of Spin: what’s on the other side of the arch and just what are the Hypotheticals. I think the first could make an interesting story all of itself, just following a series of explorers further and further through the worlds, but that’s not what Axis is. And so far as the second–we don’t really learn anything new. There are hints of something bigger (which is saying something, given something on a scale with the entire galaxy) and a few smaller neat ideas, but nothing quite comes together.
All together, I think one can either read just Spin or read the entire series (since from what I’ve read, Vortex is more of a sequel to Axis than either was directly to Spin). Looks like we’ll give Vortex a chance.