Reducing overall calories usually works, in losing weight. Diet shakes can give you a helping hand, but it is important to ensure you get a balanced diet with enough nutrients to avoid deficiency. It can also be a waste of money as some of these products are way more expensive than they should be, often to allow for big profits in their network marketing scheme. There is a lot of money to be made off people desperate to lose weight. You might be able to make healthy, low calorie shakes at home.
Another thing to keep in mind is learning how to make permanent changes to the amount and type of food you eat, so you don't regain weight after stopping the shakes. Personally I am wary of ultra low calorie diets, such as less than 1,800 calories a day. I guess 1,200 a day might be ok short term, but some people eat only 600-800 calories a day, and I just don't see how that is healthy. But each to their own I guess. Like I said, it's just my personal opinion, your mileage may vary.
Also note that rapid weight loss increases the risk of gallstones in people who have risk factors for that. A strict diet will usually see 2-3kg weight loss in the first week, which is mostly water. As a general rule of thumb I think any more than 1kg loss per week is too fast.