This past weekend, my fiancee Liz and I bought a spray bottle so we could use it to train the boys to stay away from certain places -- the diningroom table, the power strip under my desk, etc. We've tried several other cat repellants and each one either doesn't work or only works to a point. I'll post my reviews of some of these soon, but we've tried a noise-based solution, a motion detector that sprays water, and sticky doublesided tape. We need to be able to train the boys not to go to certain places.
That said, I have a hard time motivating myself to spray the boys with water. I always feel so guilty about it because it startles them so and they usually jump away with such wildness that I worry they will hurt themselves. Tonight, I saw Beowulf on top of the TV set and I sprayed him. He jumped off so fast that he fell on top of a speaker with a loud crash. He seemed ok physically, though I'm not sure how I'd know if he broke a bone. But he was obviously startled and upset. I gave him some treats and he calmed down and went to sleep, but I feel so guilty.