A lot has been said here. For me at least, XP is still a stable and worthy work environment, and probably has had the widest hardware and software compatibility of any modern Microsoft operating system.
While I would argue that XP SP3 is much different than XP SP0 / RTM, at its core it is still NT 5.x. There aren't too many operating systems that can still be used on a daily basis after 16 years, and many third-party software and hardware manufacturers are still supporting XP, as long as it is SP3. Some examples are HP (any printer you pick up will support XP SP3), Norton/Symantec antivirus/security products, Linksys and Netgear wireless devices, Logitech keyboard/mouse products, etc.
There are many other manufacturers continuing support but I have made my point. In my opinion, if the system still does what it was designed to do, it is not obsolete, and continuing operation is not an issue.