Hyper-Threading in action
Using a Hyper-Threading enabled system looks, to the user, like using a dual-processor box. Windows' Task Manager shows a pair of CPUs, like so:

Task Manager's dual CPU use indicators set a geek's heart aflutter

The picture above is fun, but it also illustrates an interesting phenomenon. The task you see taking up 50% CPU time is a Folding@Home distributed computing client, and as you can see, it's oscillating between the systems' two logical processors, just as it does in SMP systems sometimes. The penalty associated with the required context switches here isn't as great as it would be with SMP, because cache and other resources are shared between the logical CPUs. Still, I'd rather set the processor affinity for the Folding client and keep it nailed to one logical CPU. Like this:

One logical CPU runs at full tilt, while the other is idle

By the way, the 50% CPU usage number you see in Task Manager doesn't mean that 50% of the processor's execution resources are still available for the second logical CPU. The OS only sees two logical CPUs, so it can't give any deeper insight into what's going on under the hood.