I should really hate this book. It's a travel-into-your-own-past time travel story. Worse than that, it actually devotes a noticeable chunk of the story to trying to explain away the assinity of the Time Plot with the argument that time is too complex for our puny human minds to comprehend mathematically, so we can forget that a paradox is logic's way of telling you you fucked up in setting up the problem. (The assumption of time travel leads to paradoxes, therefore the assumption is false, end of discussion.) Varley almost goes for the predestination explanation (time travel into your own past creates no paradoxes if all of time is an immutable predetermined lump; you can travel into the past because you always did, but you don't have free will to change your mind), but then apparently has characters choose to do what they must to avoid paradox. But having the choice is completely untenable, whether you exercise it or not. I should have thrown this book across the room in disgust, but I didn't. So I'll stop ranting about time travel and get back to the book.
This book manages to succeed because it's humorous and lighthearted enough that I didn't have to be annoyed by the time travel. It's a nice bouncy adventure story, with a plot twist that really surprised me, but made for a much better story than what I thought I'd seen coming. I think Varley is trying to slip in a message about the true intelligence of the supergenius and the true wealth of the superbillionaire, but to consider these things would be to find the book serious enough that the time travel was unforgivable. So I'll let myself consider it a fun piece of fluff, and give it an 8 out of 10.