"and so two threads can be executed simultaneously"

And that's the mistake most people make when it comes to hyperthreading. The two threads on the logical cores don't run simultaneously, but alternately. Each logical core is basically only a set of registers.

The plus in hyperthreading is twofold. First the OS need to do less context switching because more CPU threads are available and context switching is expensive. Second when on thread has to wait for cache, I/O or other things the other thread can utilize the physical core. Maximum gain i've seen is 120%-130% CPU usage on the two cores.

Hyperthreading is OK in a low utilization environment with many threads competing for CPU. In a high CPU environment (such as OLAP) it can even start to work against you as funny scheduling artefacts start to occur. I've seen HT machines decrease performance compared to the same machine with HT turned off with heavy workloads. Even on Intels newest CPU's.

Another thing to watch is that the CPU high water mark is not 100%, but 60%-65% with HT, and you don't know exactly where it is. With CPU target normally 70% or below, this becomes 45% or below with HT turned on.

HT is good stuff on certain workloads and bad news on others. Be sure to benchmark HT on and off for your particular workload.