I've heard a lot of criticism on hyperthreading on various lists. While I haven't actually measured the performance of a machine that has it enabled vs. disabled, I have noticed that the same machine with hyperthreading enabled responds much faster to SSH logins etc. when the machine is under heavy loads. Given that I've had no problems with using HT, I'm happy to have it.
Absolutely, if it works on given hardware then use it, any performance gain is worth having :)
But if you look objectively at the gain, it is not 2x the performance ... maybe 1.25x ... maybe a little more ... maybe not.
My experience is pretty much the same, the increase in performance varies - usually around 10-30% (depends on how well optimized programs are), but it greatly improves responsiveness (the simple fact that when one program is using up all of the cpu there is still that 20% left for other tasks - and 20% of 2GHz CPU is still quite a decent amount of horsepower)...
Cheers, MaZe.