I've got the S2-1, and it's been fun to play around with so far. It does kick a good bit, but I haven't found that to be a problem really.
First the bad points.
The trigger was horrid hard to pull at first, but it smoothed out after a couple hundred pellets (and a dot of moly paste) to the point that it's now actually pretty nice. In fact, it's probably nicer than my stock P17's trigger even though it's only a single stage.
The front sight needs about another 1/8 or 1/4 inch of height as it shoots about 4 inches high at the moment. Replacing or raising it is next on my list of modifications for the little pistola.
The stock is due for a complete reshaping and refinishing at some piont as the trigger guard cuts into my finger when shooting right now. In fact, I usually remove it before starting a session. I'll probably do the big blob of putty and squeeze it to confiorm to my hand trick. Or something, anything.
The breech seal needs attention as it leaks air like a sieve. I'll replace it at some point with a new leather one or with an o-ring of appropriate size and shape. Or, I might just shim it and see what happens. Right now it only shoots about 300 fps, but I expect any improvements in the breech seal will improve that number considerably.
Now for the good points.
The thing is built like the veritable tank and to a much better level of finish than other cheapy gun, a B3-F rifle. Of course, no one will ever mistake it for a quality piece like a BSA or a Weihrach. Inside, the pins are all peened so that they don't fall out, and it's easy to disassemble and put back together.
The piston seals is leather, so it should be very forgiving and easy to replace if necessary. I expect that it will respond very well to a straightforward deburr and lube tune as the interior is sort of dry and honky. Hopefully, that will improve the recoil as well.
It cocks fairly easily, so shooting sessions are not too much of a workout. And, it's pretty accurate even with all of the things working against that right now. IIRC, I have seen 10 shot groups of about 1/2 inch at 5 yards when rested. So, the barrel appears to be well-done.
And of course, the price is awfully hard to beat ($10 from compasseco back before Christmas). At that price, it serves as a perfect training piece for the aspiring gunsmith -- after all, if you break something, you're not out any real money.
In short, I like it, probably because it needs so much TLC on the front end. :-)