I have found it very stable, going on 3 years. I am still on the original install of it, have not had to reformat and reinstall.
Whether it be xp 32, media center 2005 or xp pro x64 I believe alot of perceived OS problems are blamed on the OS when in reality the problem lies in the operator or poor setup/installation of the software and the computer settings.
I am running all 3 of the above OS plus Vista on my network and I would say that the xp pro x 64 box is probably the most stable of them all. But that is just my experience.