The salient features of hyper threading are:
i) Improved support for multi-threaded code, permitting multiple threads to run concurrently.
ii) Response time and improved reaction, and increased number of users a server can support.
According to Intel, the primary implementation only used an additional 5% of the die area over the "normal" processor, yet yielded performance improvements of 15-30%. Intel claims up to a 30% speed improvement with respect to otherwise the same, non-SMT Pentium 4. Though, the performance improvement is very application dependent, and some programs in fact slow down slightly when HTT is turned on.
This is because of the replay system of the Pentium 4 tying up valuable implementation resources, thereby starving for the other thread. Though, any performance degradation is unique to the Pentium 4 (due to several architectural nuances), and is not characteristic of concurrently multithreading in general.