Having bought a very good quality De'Longhi bean to cup coffee maker, I wanted to make sure it was kept in the best of health and decided there was no better way to do that than use the manufacturer's descaler/cleanser (as they recommended). It is possible to get cheaper descalers and they may work just as well, but I decided not to risk buying the 'wrong' sort - the brewing units and other internal parts can be damaged by unsuitable descalers, which will not only potentially void a guarantee but possibly also result in a machine no longer working ... not something I wanted to risk. On that basis, I wouldn't automatically use it if you have a different brand of coffee machine without checking suitability with the manufacturer first.
The pack contains two treatments so, depending on the hardness of your water and how many cups of coffee you consume, one pack should last between 2 and 4 months (or so). We have soft water, so a pack lasts a little longer than that, even though we have quite a bit of coffee.
De'Longhi also produce descalers in a larger bottle format - these work out a little more cost effective but not by much. I decided to use the sachets as they are measured ready for use.
Once the descaler has been through the coffee machine, someone at De'Longhi customer service helpfully suggested I put the solution into the milk jug and use it to clean through the pipes and nozzles; it works a treat.