I have tried many diets and I had alli, but my head was not really in the right place at the time, and I did not change my eating as much as I should have. They are not a quick fix and if you don't make real changes they are a waste of time and money.
Alli they are supposed to stop you from eating high fat food as if you do you can have side effects. They stop a third of the fat being absorbed from what you eat.
I think for alli (and diets in general) you have to change your habits, you cannot just eat burgers chips pop an alli and expect to lose weight.
If like me you comfort eat because you are bored, it seems like its tea time, or are happy or sad, then you need to try and address that. It is not easy, but I have managed it. I was obsessed with food even when I was on a diet.
I have been doing a meal replacement for 6 weeks and something has clicked, also I no longer crave fatty salty sugary foods. I have gradually increased my food intake. Learning to cook fish, quorn and make interesting fat free tasty food has helped me too. Now instead of salt and fat I use chilli, herbs, spices, stock and my meals are delicious. I have gone from 17 stone to 15 stone (so far)
That said if you are determined alli should help you lose weight quicker but I don't think its essential. Better to address why you eat. And have a healthy low fat, salt, sugar diet and drink lots of water.