I've had my car for over a month and have only turned it off once or twice.
For the most part, it works fine, and when stuck at long traffic lights for 2-3 minutes, you can see where it would be useful.
The only time I'll disable it is if I'm in bumper to bumper traffic and the flow of traffic is moving at almost the same interval that the ASS kicks on (IE stop for 3 seconds then go). A few times I've had some weirdness where I've lifted off the brake the second the ASS kicked on and it did a weird stall out dance.
Also, to say you should get a cheaper car if you want something fuel efficient is kind of idiotic. The 3 is one of the most efficient cars in the class and is one of the main reasons I went with this car over the Audi (20k miles a year and the delta adds up for a car that can get 30+ on the highway)
Current: '15 M235i Vert
Previous: '13 335i, '08 TL-S, '00 Corvette