Things I liked: Quality - craftsmanship, accuracy, in fact there's not much about this rifle that I don't like. This was one of my first ever Air Rifles from back when I lived in the United Kingdom. I got my first HW77K about 19 years ago and I got it second hand - the accuracy of of this thing is amazing. Mine was fitted with a Nikko Sterling 4-12 x 56 Ruby Coated lens telescopic sight and with it, I could hit a soda bottle top every single time anywhere from 10 to 35 yards away - I used to hunt rabbits with mine an consistently got head shots every single time. This is a beast of a rifle with "out of the box" accuracy that's difficult to match.
Things I would have changed: The ONLY two things I would change is to thread the barrel for an airgun silencer (not legal in some states but OK in others) and I would swap out the spring for one of them gas piston systems for slightly smoother operation.
What others should know: If you have the opportunity to get one of these then don't think about it - it's a no brainer, just buy it - you will not be disappointed