I had very bad experience after buying a cheap torx set from amazon. The screws inside the N9 are very soft and strip very easily. (The first two that you remove, which are black, are much tougher I think and I had no trouble with those.) I could not remove many of the inside screws with the cheap set so I stopped trying before most of the screws got damaged. Then I purchased a Wiha torx set. I was also planning to purchase tools for carving the stripped screws but fortunately the nice Wiha driver could (with some care!) remove even what I thought were completely stripped screws. This is a common problem with taking apart the N9 (I saw it on another thread) and it can be avoided by purchasing a quality torx driver. I think that because the screws are soft, the driver has to fit very precisely. Also, it may be a good idea to purchase an extra set of screws, and not keep reusing the same ones because after repeated take-aparts, they are likely to strip.

Edit: see this thread, which did not yet have the needed detail when I had problems -- I hadn't realized there were two types of Torx drivers, so it's not just the quality of the tool that was my problem:

http://talk.maemo.org/showthread.php?t=87086&page=3

But it still may be that a Wiha T4 is useful for partially stripped screws.