Intel's HyperThreading looked like a really good deal, initially. And I must admit that in some applications it really was a help. My favorite example was encryption/decryption on a file server I set up.
Turns out that in at least some heavily threaded software, such as Microsoft's SQL Server, HyperThreading is a big liability and can actually limit performance. Who would have guessed?
The link above is to an article at ZDNet UK. More information can be found here.