Hiya Mel,
The Alli pill will only work if you stick rigidly to a low fat diet (the same sort of diet that SW gives you). If you're having trouble losing weight on SW, speak to your consultant or post a food diary in the relevant forum - it might be that you are making a mistake with the plan without realising, or one type of 'day' suits you better than others.
Alli may help you lose more weight than you do on SW alone (they say for every 2lbs you lose through diet alone, you may lose an extra 1lb - so your weight loss would be upped to 9lbs instead of 5.5lbs if you'd been taking it). However, these results are not guaranteed, and they seem to be partially due to the psychological effects that it has - having diarrhoea running down your legs after 'cheating' with a high fat item does certainly dissuade me from eating such foods...
In my opinion, it's an expensive choice - if you're that overweight and want to do something about it, speak to your GP as the same drug can be given on prescription (obviously a lot cheaper than buying over the counter), but otherwise sticking at SW would be cheaper I think!
Feel free to ask us for help - there are some very knowledgeable people here very willing to assist in any way they can...