These caps are helpful, and do allow you to get just about every drop out of a container. But there are some inherent laws of physics that prevent them from working perfectly, especially if the liquid in the container is more viscous, as, for example, with hand lotion.
The viscosity of hand lotion creates a situation in which air is not readily "sucked" back into the bottle to replace the lotion as it is used, so the bottle needs to be squeezed harder with each application. Eventually the bottle would be squished almost flat, but long before that it starts to suck the cap closed immediately after pulling it open. Annoying, but you can usually make things work with some fiddling.
However, the cap is not leakproof, at least not along the shaft of the valve. After about a dozen applications, lotion is leaking around the shaft of the valve, and eventually becomes a mess that's not easy to clean off. Maybe I've just got a leaky valve, but after a complete disassembly, thorough cleaning, and reassembly it's still leaking a little.
So, to recap (hah!), these caps are great for use on most bottles, as long as the liquid therein is not too thick. Still worth the investment, I think.
