My first IF marked my transition to "fat burning beast" (I love Mark's term...); it was about 3 weeks in, but before that, I didn't feel like I could have done it comfortably. I remember being amazed that I didn't feel hunger like I thought I would--it was there, but not insistent--and that I was able to exercise in the fasted state. Such a feeling of liberation from the gotta-eat-every-3-hours routine I was so used to for so long. I think it can really help with fat loss, but to echo Mark's advice, don't try it if you're really stressed or not sleeping enough, because it will just add another stress on top of everything else.
On the fat loss question, keep checking the fit of your clothes--you'll know it when it starts to come off!