Just wanted to stick my oar in and say that if you're doing things right, and sticking to a low fat diet while taking Xenical, you won't ever truly 'know' that it's working - except that your weight losses should be slightly better than if you weren't taking the tablets.
Xenical stops a third of the fat in your meals being absorbed by your body. This obviously helps you to lose weight because your body can't use or store that fat. Obviously, it does also stop you from being tempted to eat fatty foods, but the fat-blocking properties of Xenical are its main purpose.
When you use Xenical properly, you don't see side effects (or see it working) because the amount of fat passing out in your stools is relatively small. I've used this example before but it might help to clarify...
Say if you ate a meal containing 9g of fat (well within the 15g per meal rule) you would pass 3g of that fat in your stools, and 6g would be used by your body in the normal way. However, because 3g is a small amount to pass amongst your other bodily waste, you would not see any side effects.
So say if you ate a meal containing 60g of fat and took your Xenical, 20g would pass through your body and out in your stools, which is obviously quite a lot, and you would be more likely to have the 'Tango' experience. That's why you shouldn't take Xenical and continue to eat high fat food like take aways.
Really it's just a helping hand to help eliminate a small amount of fat, and to help you stay on track and not get tempted to eat the fatty foods in the first place.
The rest of the hard work is up to us unfortunately!!