I love them. (I have all three.)
Good science* explanations combined with good story, and neither overwhelming the other. And as I think the books themselves say, they're not a 'how future science might actually be a bit like this', it's more 'the science that Terry has read and included in the books and you probably didn't notice' type stuff.
*err, as far as I can tell, it's good anyway. I mean, I can understand it, and it seems pretty complicated stuff at first glance. On the other hand, damn it Jim, I'm an archaeologist, not a sciencetist. And there are three of them, so presumably quite a lot of people though the first two were worth buying.