I refuse to put anything containing water inside my guitar. The case mounting system of the Oasis works, and works well. The design of the humidifier itself is fantastic - one look and you know it needs to be refilled. I have about eight of these little buggers, and I've had most of them for about three years. None has ever failed me.
They contain a crystalline powder that absorbs water. When you fill them, the water turns to a gel, so even if it breaks, the gel keeps the water from pouring out. You can buy more of the powder in case you ever spill it.
I actually keep the hygrometer on my desk next to one of these humidifiers. I fill the one on my desk the same time I fill the ones in my cases. When I see the one on my desk shrivel, I know ti's time to fill all the cases. FWIW the one on my desk always shrivels faster than the ones in the cases, so it seems to be a good system.
Here in NJ, relative humidity ranges from 90% in the summer to 10% in the winter. As soon as it gets below 40%, I start filling the Oasis tubes. When it gets above 60%, I pull them out.
I always fill them with distilled water using the included syringe.
So in short, they work, they're easy to use, I've never seen one leak, and I'm a fan.