I use a simulator at local Golf Galaxy (also happen to work there part time) and it seems almost dead on to me, but your mileage may vary. We have a couple in the store, the old school enclosed one and a newer model that I can't remember the name of but it is simply a little box that sits next to the tee area and links to a hard drive with a big screen monitor that does it all. I prefer the new style one rather than the enclosed area with the projector-type set up where you hit into a fake driving range background. The one I use is almost identical to the distances I see on the course, unless there is wet/wind/dry/etc. on the course. I'm sure you can find a decent indoor simulator at other retailers, too. Don't be wary of them!
It's actually funny to see all the customers who claim to hit "300 yard drives" really hitting it about 250 with the latest greatest drivers and blaming the simulator.
Scott