If you Google e46 crack or e46 subframe crack, you will find dozens of posts. There is a 3-part video on YouTube on how to repair the problem.
The pictures on the web and the videos are mostly M3s that people have thrashed. One of the YouTube videos is a 330. As I mentioned, I own two e46s, and I know several guys at work who own them and my sister had one. 325s, 330s, and Ms. All are daily drivers, no track cars, and I have never personally seen one crack.