I got one of these in the post today.

t was a good price as it was losing all its gas as soon as it was put in. As it turns out this was just a washer on the valve stem spring screw instead of an o ring which stopped the valve body from making an effective seal. (At least I assume so cos that's the only part I replaced and now it works.......)
Its a joy to shoot, comparable to Snoopers LP10 which I tried in April in operation, which is nice!

The only petit-bijoux fly-ette in the ointment is that's it's only achieving 300fps with AA field (the only .177 pellets I have) with massive shot to shot differences in the 50fps range. This points to a sealing issue around the breech, but that's a job for another day.
Does anyone have experience with these? Anything else I should check for?