Great water reservoir, no issues with the zip-lock top
I bought two 6-liter tanks for a backpacking trip on the Channel Islands three years ago now. A hell to carry but really worth it, not much water there.
Anyway, I've used them a lot since then just for car camping mostly, and have never had an issue. The zip-lock style top closes easily for me, and as the recommend I flip it upside down to make sure it's properly closed, and I usually don't see a drop.
Finally, as some users have mentioned, it is compatible with the rest of the platypus line which means you can screw a platypus hose on it, or probably their filter although I haven't tried.