Hyper threading?

Hi all,

I searched list archives, but did not found anything about HT of Pentium 
4/Xeon processors. I wonder if hyperthreading can boost or decrease 
performance. AFAIK for other commercial servers (msssql, oracle) official 
documents state something like "faster, but not always, so probably slower, 
unless faster". User opinions are generaly more clear: better swhitch off HT.

Do you have any experiance or test results regarding hyperthreading? Or what 
additional conditions can make HT useful or pointless?

TIA,

Mariusz