I have been using Microsoft Windows Vista Ultimate 64-Bit for more than a month and I must say I am pretty comfortable with it. Even though not much changes from Windows XP besides the GUI, it supports DirectX 10 which is a must have for all gamers.
Till now, I have gotten only 1 Blue Screen of Death (BSOD) when I am upgrading to the Windows Vista 64-bit service pack 1. Since then, I have not gotten a single crash *TOUCH WOOD*.
Microsoft Windows XP Service Pack 3 is due to be out tomorrow (29th April 2008), but apparently the file it is already on the Windows Update server. If you are suspicious of the authenticity of Windows XP Service Pack 3 file posted by 3rd party sites, this is the link for you as it is directly from Microsoft.