I've got a couple and shot plenty from .223 up to .338 WM.
Most are very good and accurate, but there was a period in the 80's when they were having some problems with their barrels. They had their barrels contracted out to an outside company for production and it caused inconsistency. From the mid 80's on I believe this changed and their barrels were much more consistent and hammer forged in house.
I've got a 7Mag that shoots MOA, but I've got an .06 that shoots just outside that. For 99% of hunting applications they are just fine, I just wouldn't set one up for shooting longer than 300-400 yards.
There are guns with better actions (probably not stronger), better triggers, better barrels and better accuracy, but the M77 does a lot really well and if the price is right owning one is a good option.
My only issue with them is that the stock doesn't fit me well. When I shoot my 7 Mag the felt recoil is much greater than some of my much bigger calibers due to stock fit and shape.