The problem with exercise is the minute any meaningful weight loss kicks in, so does The Hunger so you need to have a lot of self control to keep taking in the same amount of calories while upping the activity levels.
In my opinion, what exercise does is help maintain a steady weight or at least minimise the gain - I think it was the Harvard Nurses study that found that women who cycled regularly to work gained less weight over a couple of decades than those who didn't.
From my point of view, it's the ability to eat the odd slice of cake when I fancy it that motivates me to cycle, walk or run more than I might otherwise. I did calculate that an 11-mile bike ride was roughly equivalent to two slices of toast with butter & jam, which puts it in perspective!