I owned one a while ago, I saw it at a bootsale for a couple of pounds on a wet camping weekend and brought it for fun.
I can't speak for those particular models but the one that I had looks cheaper and nastier so the ones that you've seen will probably be better. It "did" allow me to hear things from a distance that I couldn't hear normally, but it wasn't spectacular.
If somebody was standing a hundred or so metres away and I couldn't tell that they were saying anything with my ears, it did allow me to just about make out what they were saying, but it wasn't anything like you see on a James Bond movie. They are basically just amplified microphones. The dish does provide a little extra amplification but it's probably 50% for show. They are also line of sight only.
With one of those Chinese ones, I'd guess that you could probably stick one out of your upstairs window and hear somebody talking down the street. But you'd look like a total fool doing it.
Basically, they do what they say that they do, but they're more toys for fun than a serious tool.