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The only serious defect I've seen in this OS is its inability to deal with very large files.
Moving >5 GB files around is a pain because it will insist caching them in memory.
Still, XP64 (or its server counterpart, WS2003) runs circles around any other Windows version released so far.
The responsiveness of XP 64 bit on my E9300 machine with 8GB of ram is very poor compared to 7.
On a modern 64bit computer there is no compelling reason to use XP64bit unless you happen to develop .NET 1.1 applications.