A few things about simulators...
- They're computer-driven devices, so they have to be calibrated time to time to make sure they're simulating accurately
- Some golf shops "soup up" the simulator to make you think the club will deliver more yardage than it actually will. As a test, always get baseline data with your current club. If you're suddenly carrying your driver 250 yds., when you only get 220 yds. on the course, maybe the machine is off a little.
- If possible, bring along a couple of sleeves of the ball you play on-course to use in the tests
- Make sure you warm up first. Do some stretches - if possible, hit a small bucket at a nearby range - and then warm up a little when you get to the shop. If you get out of the car cold and start hitting full shots 3 minutes later, it probably won't accurately gauge your swing.
The local Golf Galaxy actually cautions us that their simulator measures things "a little short." I find I hit iron shots about a half-club longer outdoors than I do on the simulator.
Besides the machine, you need a player's rep who can help you interpret the data (unless you have a personal Trackman in your back yard, and just didn't bother to bring it along).
Also, a formal club fitting takes longer than just comparing Your Driver vs. New Driver for 10 minutes. A formal fitting should allow you to hit different shafts in the club(s) in question.