I loved my first-gen X2. Back then, I probably never would have suspected that multiple cores would have helped me out, but you never realize how much crap is happening without you actually seeing it until you have all them cores to feed!
Although, my lordy... they were expensive!
AMD Athlon 64 X2 4400+ Toledo 2.2GHz Socket 939 Dual-Core Processor ADA4400DAA6CD - OEM
$615.0
I know some unhappy people who owned the 939 X2,
first X2 were quite expensive compared to single core CPUs, not a long time later Intel released the Core 2 Duo (when Dual Cores started to really get relevant) which was just far superior, and the AMD pricing was just bad, even more with AMD abandoning 939 and going for AM2 around the same time...
while Intel released some cheap e2xxx and e4xxx later which could beat even the fastest K8 if you overclocked it...
I think the original K8 was a lot more exciting than the K8 X2.
as for me, I bought one k8 X2 Brisbane g1 (when it was already quite cheap, but I regret not buying a e2140/e2160 instead), it was awful to overclock, slower than windsor, and I ended UP upgrading to a E5200, now that was a great little CPU, almost as good as the XP 1700 in my opinion, you could get 4GHz and stock e8xxx level of performance for a lot less.
cheapest AMD dual core at the time Athlon 64 X2 4200+
$537
cheapest Intel dual core
Pentium D 820
$241
one year later you could buy this:
Intel Core 2 Duo E6300 1.86GHz 2MB $183
and it could easily beat this:
AMD Athlon 64 X2 4200+ 2.2GHz 512KBx2 $240
AMD Athlon 64 X2 3800+ 2.0GHz 512KBx2 $169