To follow up on this: I finished the book on the way back from Montreal, and had a while to think it over by now.
@Robbt's statement that the second half of the book is less brain dump and more plotted sci-fi is actually true, and for me that made the second half much easier to get through. Although @Dominic is right, too, that the whole upload plot is really just a pretext for conflict. That part of the plot is the least relevant to anything, and scientifically also waaay out there, so whatever.
Overall, my impression from the 50% mark holds: the book introduces/develops/explains a ton of interesting material, but it is lacking in other important ways. I agree with @Alanna that the non-issue style of displaying the relationships is one of the most powerful utopian moves Doctorow plays, normalizing (today's) non-mainstream lifestyles pretty effectively. It's really just the actual sex scenes that I take issue with. Not because of what's displayed. I just think he doesn't do a very good job at making it believable; it comes across more as fan-fiction written by a teenage boy. Which is fine, not every writer needs to be great at everything, but IMHO, it would have been better to just imply the entire scenes, or axe some of them entirely, especially since none of them actually seems to advance the plot at all. This is especially true for the scene between Iceweasel and Nadie. That scene just didn't have to happen, at least not on the page, since it had no consequences for the plot at all. And if it was there to make a meta-point about the normality of homosexuality, a "kiss, fondle, next scene" would have been enough.
If you think (like me) that this makes me sound a bit like a prude: it's not constrained to the sex scenes. Many of the "brain dump" dialogues throughout the book feel very staged, unnatural. I get that the purpose is to explain this or that concept to the reader. But if I can tell that that's the (only) point, for me that's already a sign of lazy writing, and it makes me feel manipulated. I'm okay with a book (or any other medium) being subversive/manipulative, in fact that was one of the main points of reading this book: being confronted with new ideas, and hopefully learning something in the process. But if you're gonna present your ideas as a novel, better keep me entertained.
Bottom line: I'm glad I read this book, because it did contain very interesting ideas and things to think about. I just wish Doctorow had made a better book out of them...