This is a link to an old THG article that does one of the best jobs of explaining HT.http://www.tomshardware.com/reviews/intel,557.html
Hyperthreading has improved since that article.
The Prescott series P4 intro'd HT2 than later Xeons utilized cache queing which reduced cache thrashing and made HT even better.
The way software is written currently you are definitely better off w/ HT on.
In the beginning in 2002 some apps would suffer due to HT but that is not true of newer better written software.
In a single threaded app you will not see a performance gain but as already stated you will be able to run other apps at same time more efficiently.
Early HT paved the way for better written software for multi-core systems today.