LIke some others, I didn't think I'd like the ACC, but it was part of a package that I did want at the time. Now that I've experienced it, I would buy it on my next car as well. The i3's system utilizes just a camera, which can have some contrast issues in some lighting conditions. Some others include LIDAR or RADAR sensors, are more immune to those visible light issues. But, all in all, I think that it is worth it. If you're road has lots of curves or hills, and the traffic isn't heavy, it does take it a moment to recognize a vehicle ahead in your lane since it may be out of sight of the lens. It's not an autopilot. It does work fine most of the time, and if desired, you can cause it to revert to a 'normal' CC. Roads with trees right to the edges during bright sun can go from bright to deep shade, and that confuses it as can low sun angles where it overcomes the ability of it to develop enough contrast to identify things. IMHO, you just need to actually drive your car, the thing is an assistant, not a master.
2011 535i x-drive GT, 2014 i3 BEV