They've worked great for me. Each core is twice as fast as the Pentium 4 it replaced and there's 2 (or 4 with the quad). Power consumption wise it's about even with my last Pentium 4 machine. That's 4x the CPU power for the Duo and 8x the CPU power for the quad for the same energy consumption/heat/noise footprint. I have had issues getting XP performance out of them (and the quad core 6600) when running backtest processes under Fedora 6/7 Linux, but that may be a java issue.
Thanks You mean you have had issues fetting speed out of them or do you have crashes? Only reason I want to use xp is eSignal