Copy and paste from a thread I posted on last week.
I've been using it for 2 weeks now, had great results with minimal side effects. I've even pushed the fat content, although the day after I had fish and chips wasn't exactly great. Even still, I've never been caught short, or had any accidents, so I'd class that as a definite plus. In a way I'd prefer it if I did get more side effects, so then I definitely wouldn't sneak stuff in like fish and chips (even if it was a half portion and lightly battered).
I'm combining it with a low calorie/low fat diet and a lot of exercise. I try not to go over 25g of fat a day, and my calorie limit is 1600. As well as the usual running around after a 2 year old and walking, I'm doing the 30 day shred which is HARD work, but I'm already seeing results.
I think Alli is a really great product, but you have to combine it with a low fat (specifically) and low calorie (ideally) diet, and a bit of exercise. There's no point in getting it unless you're really committed to losing weight though, as it is expensive and not something that should be taken lightly.
Because of the success I've had with Alli and the 30 day shred, I'm revising my weight loss target, aiming to lose another stone on top of the 2 I've already lost.