My m15x. Hmmm.. The screen is bright and vivid, I usually run it at 4/5 brightness. The mousepad turns off with an external mouse, which is nice. The alienhead is easy to turn on. The screen on mine, along with all m15x's is VERY SQUEEKY when you open or close it. It is also wobbly. There is a moderate amount of keyboard flex by the arrows, which is not too bad. When I play cod4, the fans are always on, but that is because I have the 8700 graphics card, and no cooler. I would give my laptop a 4.5/5, because of the minor problems.
People's m15x early had a lot of problems with the 8800 graphics card downclocking. This was fixed with a bios update, installed on all new alienware laptops. People's m15x also had problems with the lefthand side of the notebook "peeling up". My notebook has not had any problems like this. Other than that, I cannot think of a major problem with anyone's m15x.
Also, if the m17x is matte, which I can't remember, you must never scratch it. People's m9750's got damaged over a few months, and the finish peeled a lot.