I think you're right about the machines. I'm tall and thick so they're more accurate for me, bur I've heard that complaint from other people.
I have yet to find a good place to stick the watch when I have boxing gloves on so lately I just don't use the monitor for that.
I think I'll try it swimming and if it breaks I'll send the company a letter askng for a new one. It says "water resistant to 100 meters" but in my experience that's always a gamble.
Yeah, I found that I workout harder when I wear the monitor than when I just go based on how I feel. It helps push me further.