Just to repeat much of what others have said.
The QBs are a poorer copy of the classic Crosman 160 and its variants (like the lovely 180 carbine). The Crosmans are excellent, except in the scope mounting department, and many in the US even today tune and modify them to be even better.
The very first QBs (the 77? 87?) for the US market were by all accounts really well made by the Chinese, but the manufacturing quality was quickly brought down to hit a lower price.
That said, they are still good value new. And at auction you can sometimes pick them up for genuine peanuts - £25-50 hammer price.