The company I used to work for set up a gym and hired a team of personal trainers to come in and work with us in small groups. They let me continue the program even though I'm no longer an employee. Anyway, the instructors for both the groups I go to (one is more curcuit, one is more strength) are lead by women.
One is a bit more seasoned then the other and really knows what she's doing, but I don't have any complaints about either of them. Plus, they are much, much less distracting. When one of them is showing me an exercise, I'm actually paying attention to the exercise!
Although, now that you mention it, I think I may work a litle harder when the instructor is a man. Must be one of those macho, competitive things. Huh.