I did the GI Diet after my son was born and lost around 35lb in 12 weeks.
Once you get the hang of what low GI foods are out there, and learn to make better choices it becomes really easy to stick to. I think the GI Diet, combined with a food journal (the best diet tool I've EVER known) are a great way of being aware of what you're doing to your body.
Oooh another good thing about low GI foods is that nutella is included! So I still got my chocolate fix, albeit in smaller quantities than I'd like
If you're thinking of a super fast weight loss plan it might not be the way to go though, because my friends & family think the reason the weight came off so quick was because I was breastfeeding. Hope to help and good luck!!