As mentioned above, it is substantially faster than software simulation, but also more expensive. The simulation mentioned above that took 8.5 hours on a hardware accelerator would run in less than a second on an emulator, providing substantially more productivity for the design team. Emulators, however, are two to three times more expensive than hardware accelerators. Also the use model is more complicated. Since an emulator is actually a hardware model of the design it is much more difficult to debug than the software models used in simulation.
Speed comes at a price.
With a faster tool the designer can turn his fixes faster and get his product to market faster. The designer can also increase the sophistication of his test, and thus improve reliability by increasing the likelihood that no design flaws escape undetected. A good example of this would be the integration of many components onto a single chip for the purpose of reducing cost. Designs larger than 1 million gates start to bog down the workstations and may require some sort of acceleration. And, due to the increasing complexity of electronic devices, designs, of course, are not getting any smaller.