I've never bought any, but in the past several semesters, I've developed a mixed opinion of them. In MOST classes, they seem to be exactly the same as the "US versions," page for page, but I've seen a couple that were SUPPOSED to be exactly the same, but weren't. Depending on the professor, they may work with you on the difference, or they may just tell you, "Too Bad, buy the book at the bookstore and you don't have that problem."
In one class, the professor assigned "Problems 1, 3, 5, 7, and 8 on page 158", but the international versions either didn't have ANY problems on page 158, OR had different problems on the page.