::ahem:: Hello. My name is Nick, and I am recovering Leslie switch addict. It was hard at first, but the more I pay attention to the sound, the more I know when the best times are to flip that switch. It's really important because it affects your sound.
Also, the less you move that switch back and forth, the more distinguished it becomes to the audience. If you're flicking it all the time, then no one will really understand its value. When you use it at just the right times (like fast to slow right before you hit that bangin chord you just came across last night at 3am...), you'll make more of an effect.
Also, playing slow filler chords and ballads on slow with no chorus/vibrato has a great, smooth effect to it. I think it is more pronounced, also, to the audience when the Leslie is miked.
