I recently got into paper terrain so that I could quickly play with some new 15mm sci-fi miniatures. Of the numerous sets of paper terrain I've bought or found for free on the internet, Zone 10 is one of the best. There are plenty of interesting shapes (not just blocks) and the artwork on this set is quite nice. The pieces go together easily and don't have a learning curve as steep as some other paper terrain products out there. The pieces are stackable, mix easily, and there are enough little detail pieces to make each building unique, if you want to take the extra time.
The price is definitely right! I also like that this site allows you to preview the images so you know exactly what you're buying; this is not the case with several other paper terrain sellers online.
The only problem with this set is that it's scaled for 10mm. I thought that they're actually fine for 15mm (at least with the Khurasan Miniatures I have), but it seemed an odd scale to choose, especially since one of the beauties of paper terrain is that it's infinitely scalable. You can buy a "28mm scale" paper terrain set and then just print it at 50% for smaller scales. This, unfortunately, doesn't work in the other direction, at least not with this set, where each building has its own page and the walls aren't broken down across multiple pages.
Overall, though, an excellent set; I've already printed off one complete copy and will print more!