I have a few of them and have absolutely no complaints. They are accurate, shoulder quickly, handle well, and I've never had a jam or feeding issue with the bolt guns.
Currently I have:
Heym 450/400 Double
Heym 375 H&H Martini Express
Heym 458 Lott Martini Express
Heym 300 Win Mag T bolt
Awaiting delivery on:
Heym 500NE Double
Heym 300 Win Mag Martini Express
The Doubles are open sight, the 458 has a red dot, the 375, and 300's are scoped.
I use the 450/400 primarily for elephant and cape buffalo, although I did shoot a giraffe, finished a rhino, and shot a zebra. The closest shot I've taken was an elephant at 21' and closing, the farthest was the zebra, ranged after the shot, at 75 yds.
While it's effective and accurate at 100 yds, I try to stay in the 30 - 40 yd range.
At the time I bought the 450/400, I hadn't hunted anything larger than a pheasant in 40 years, never hunted in Africa, and was going to on a "One - Time" safari to hunt plains game and possibly a cape buffalo. The 450/400 full filled the lore of what I imagined a safari to be.
Now, 3 safaris in with another one scheduled for June, 3 elephants, 5 cape buffalo, and a rhino - I figured I was pushing my luck with that 450/400 and bought the 500.
The 375 has proven itself indispensable; it is my go to for all plains game and I knocked down a cape buffalo and the rhino when the shots were a bit too challenging for the double.
I have shot it out to 500 yds at the range, but for the type of hunting I do - that's a long shot. I'm sure the sheep and elk guys can go way out with it.
My closest shot was point blank into a hippo as he was rising out of the water, my farthest around 350 yds.
I've shot other makes, I'll stick with Heym