I took the exam in Feb 08 in Houston. The best location probably depends primarily where you are coming from. In and around the loop, the Harwin location is probably best. Now, when I took it, they had some issues with the A/C so the room was quite hot, but that was easily solved by taking my sweatshirt off. That being said, everything else was fine: the proctors were nice, restrooms easy to access and clean, and the area was fairly quiet.
One of my friends took the exam in the Clear Lake location. I think he said the place was fine, but he definitely called me at 7am to help him get directions, and that's a kind of stress I definitely do not want before a major exam. Anyway, hope that helps! Feel free to email if you have more qs
