I'd never done clipless until I bought my latest bike, which came with SPDs. Wouldn't dream of going back now. Climbing is so much easier, especially those times when you find yourself in not quite the right gear and have to stand - you can kind of pull yourself up (erm, that's a bit of a lousy explanation ).

As far as saddle height goes, I think, as mentioned, the reasoning for lowering it is to make it easier to get off the saddle when necessary. If you're not downhilling, you probably won't need to lower the saddle very much in comparison to your road bike, and once you've settled on a comfortable height, you may never adjust it again. Bike geometry and saddle width (it's quite a bit easier to get off the back of a narrower saddle) are going to affect how far you need to lower the saddle, so there's no way of saying exactly how much you might need to lower it - just get out there and experiment until you find what works for you.

Happy riding!