I purchased these last year as part of my never-ending pursuit for a reliable way to water my plants while I'm not around.
The idea is simple. Cut the bottom off a plastic bottle and jam the uncapped end of the bottle over the top of the spike. Insert the spike into the soil and fill the bottle with water. These spikes have a flow control setting so in theory you can control the rate at which water flows through the spike and into the soil.
I've used similar less fancy products in the past. They're good for getting water down into the soil near the roots which avoids soil compaction. But they're not so good as a set-and-forget tool for watering. As with the cheaper and less advanced versions, the output holes on these get clogged. The opening at the top has a filter to prevent the spike from getting clogged but of course, the filter gets clogged over time and needs to be cleaned occasionally.
The seal with the plastic bottle was excellent. No leakage. It's just a press-on fit.
When I first set these up, I set the flow knobs to maximum and they would drain a 2 litre bottle in a few hours. Set on minimum it took about a day for one to empty down to half and the other to empty to a quarter. Then they stopped working. I could play with their placement in the soil and they'd starting working again but after a month, even set to maximum flow, they stopped working completely. I guess they got clogged up. I'm not really sure because at that point I stopped using them.