If you've read my other reviews, you'll know I liked the previous novels, Spin and Axis, a LOT. Sadly, I can't say the same of this final book in the Spin trilogy (this was not entirely unexpected, having read other reviews). It didn't have the same sensawunda for me as Spin and Axis did, nor did I find it as engaging. The two 'familiar' characters from the previous books didn't feel like the same characters (which in a way makes sense, but was still disappointing), and I think the storyline(s) suffered from the switches between the two vastly different times the book follows - and also from the way the 'future' storyline was told (the reason for this becomes clear at the end, but it still doesn't alter the fact that for me, the way those viewpoints were shown felt stilted and distant). I didn't really feel connected to any of the viewpoint characters, and the most interesting character to me (Orrin) only got the briefest of viewpoint sections - not even a whole chapter.
I'd only recommend reading Vortex if you feel (like me) you HAVE to complete the trilogy, or if you really MUST know the true nature and/or motives of the Hypotheticals - and if it's the latter, be prepared for that to be something of a disappointment, too.