I've heard in a few of the reviews that the ESP is rather abrupt when it activates, and it really snaps the smart back into line. I've experienced the Mercedes product in the larger S class, where you can tell it is working, but it is nowhere near abrupt or violent... probably due to the sheer mass of the vehicle.
I set mine off a couple of times going over the speed humps in my old apartment complex - but it was just enough to make the light flash on the dash.
Yesterday evening, I was pulling out of my parking lot at work in the evening, and there was noone around and it was raining. I decided to see what it felt like (partly because I wanted to see how intrusive it was, and partly to see what it felt like so I could tell if it were ever coming on without looking at the dash... something I'd recommend if you have never felt it).
I pulled out of the lot, and making a right hand turn I punched the accelerator. The back end started to snap around in a fishtail, and after I got maybe 5 degrees off of my intended course, I felt a shudder, lost a bit of power, and the car snapped right back straight in the lane and then continued accelerating. It was nowhere near abrupt or violent, but it definitely got me straight in the lane and made the car behave. I'm definitely happy with how it performs... I'm curious to see what this winter looks like (although my smart will probably not see much snow as the VUE is a monster in the snow/ice).
Have any of you had it activate yet... either purposely or in an accident avoidance situation?