The bonus has historically been the bulk of compensation in front office IB, well more than half of your annual take home after a year or two. During the good years, these bonuses were effectively guaranteed, and could basically be seen as a lump sum salary payment at the end of the year, with all but the worst performers getting them--if you didn't get one, it was a not-so-subtle way of telling you that you didn't need to be working there anymore. Certainly the numbers vary--most places use some bucketing scheme--but just about everyone in the front office gets them. They are still a huge chunk of compensation, but much lower than they were a few years ago, with some places giving out the goose egg (or very close thereto).
IMO though, not worth it at all. The comment about "hourly wage" is correct. 100+ hour weeks, every week, are absolutely the norm.