Yeah, I've got one. It's a fine gun as they go. Obviously not a high end, $2000 gun !
Yes, the trigger is adjustable, yes, the trigger has a clean, second stage break like many higher end guns, yes the guns are accurate to at least 75 yards, yes the gun/stock is durable. I've had mine for about a year, no air leaks, no other problems. The plastic stock, it seems that it can be beat around a bit without damage.
I had two complaints, and fixed them both with little monetary outlay and a few minutes time. 1. The trigger return spring. It's very stiff. I replaced it with a short ball point pen spring. 2. The hollow stock echo's the mechanical noises of the gun. A piece of sound deadening material, stuck on each side of the inside of the stock, back near the action fixed that problem. I have no other complaints about mine.
Don't know about a regulator installation, don't care.
In my opinion, it's well worth the original asking price of $599, so yeah, $399 for a new or well cared for gun is a good price.
Mike