Definitely a must for your Lamy fountain pens, especially if you're using them daily. This converter has saved me a lot of money and makes it very easy to refill my pen with whatever ink pot I'd like. I have cartridges only for travel - since lugging an ink pot around is a bit cumbersome.
These are great if you want to use waterproof or various coloured inks in your Lamy fountain pens. I have several Lamy Safari that I fill with waterproof ink and use for sketching. These cartridges are easy to install and fill with the bottled ink of your choice.