I got the software done for free, and it's worth it for me of course. Car feels smoother but I doubt you'll notice an increase in horsepower. As you slow down, the gears change much more smoothly than before the update. Also, I feel much quicker throttle response.
If the car has never been updated before, then definitely update it. Mine was updated in 2010 and the latest update which came out around Christmas I think did change the feel of the car.