Taking on sustainable weight loss, IMO:
a) weightwatchers - ok if if teaches you about healthy portion size and healthy food choices, and you can stick this way of long term. Less good if you just trade normal bread and biscuits and their crap food (diet bread, cakes, biscuits). But if you want to focus on healthy portion size why pay £££ to weightwatchers when you can do it for free at myfitnesspal or other freebie websites?
b) Pills, personally don't really see point because you have to stick to low fat diet in any case. Why not just do low fat / low calorie diet without the pills?
c) Very low carb (ketosis) diets are not IMO sustainable. I do moderate carbs but its important to be allow occasional lapses. I worked on 90:10 for weight loss and 70:30 for weight maintenance.
d) Paul McKenna is IMO sustainable if it teaches you good eating patterns, ie to eat when hungry and not indulge in emotional eating.
e) Many people don't realise that when weight loss, its generally not all fat, but muscle mass is lost too. As a result when they lose the weight and get to target their body fat is higher than it was when they were last at this weight as muscle mass is lower. This generally leads to weight gain as the dieters BMR is lower than it should be as muscle is metabolically active and fat is not. To avoid/mitigate this it is essential to eat enough protein (ideally at each meal) and undertake resistance training whilst losing weight. Yes the weight loss will be slower but its more likely to be sustainable weight loss.
I am 4 months into my maintenance phase and my next objective is to try and restock some of my lost muscle.