I did it for a few months and loved it - only stopped because I sprained my ankle playing football one day, and then it started to rain. I only did it once a week, but it took my focus away from the size I was wearing and put it towards being able to complete the tasks in class. Worked my way up from no running at all to completing my first 10k. And I dropped a dress size anyway. 
If you are nervous at all, BMF usually holds a monthly fitness test. That might be handy to do first, as you can meet the instructors and do some running, press-ups, squats, etc. You and the instructors then both know what level you are starting at.
Everyone is puffing and moaning through the class, even the turbo-athletes, but we were usually doing things like leapfrog and bunny hops - games we all played when we were kids. The instructors I had were more like pushy high school coaches than Full Metal Jacket drill sergeants.
If you do go, don't eat right before class! Maybe a spoonful of peanut butter (or something) if you're ravenous, but save the food for afterwards.