Just picked up my 335i. Not nearly as jarring as I thought it would be. Some instances I didn't even notice. The car never shook, and the engine was ready by the time my foot moved from the brake to the gas pedal. Stuck in traffic in Stuttgart yesterday, I was at a standstill for minutes at a time. Great knowing I wasn't sucking up gas while stuck for 45 minutes.
The BMW rep at BMW Welt told me that if you're at a stop and won't want the ASS to kick in, you can pump the brake pedal after stopping once, and that will disable it for that stop. So you don't actually need to hit the button when you come to a stop but can clearly see you'll be moving again in a few seconds. I've used this a couple of times, though I may not be exactly clear on the procedure.