I’ve done AMS-JNB; my partner has done AMS to SIN, KUL, BKK, HKG, ICN and a few more that I forget. KLM long haul in Economy is excellent... nothing fancy but good seats on most 777 and A330, with fast and responsive in flight entertainment, plus generally charming service from good crew. I wouldn’t pay extra for Economy Comfort, and I haven’t had the chance for Business Class beyond Europe, so cant comment.
Our long-haul default, now we've retired, is KLM business class, usually on 777s, from Manchester.
With no first class, you do get good personal service (they remember your name!) in a small cabin and most of their planes have flat beds.
I've done MAN-AMS-JNB a few times, and although the times are not ideal (day flight southbound, late night northbound) it beats the Air France 380 via CDG hands down.
AF evidently reserves its best service for its first class customers.
We've also been to Toronto and Salt Lake City (new 787) on KLM.
Connecting in AMS on a business class fare, you get a front seat on the short leg and are first off the plane, and use of the KLM lounges at AMS.
Generally the prices are as good as you can get for a decent carrier, and often better than more direct routes.
Not so good for Australia/New Zealand though, as KLM partner with Malaysian and it's a messy journey with high prices.
Some of this is dictated by the frequent flyer scheme you are in, and I'm in SkyMiles (Delta), so usually go for their partners KL/AF.
These days, I wouldn't even think of BA, given their treatment of Manchester and the bedlam that is LHR.
One of the variables at AMS is the runway your flight uses.
If you use the furthest Polderbaan runway it's a long way to the terminal, which can make a big difference if you have a short connection.
But you can't beat EasyJet and Ryanair for cheap single-leg trips (ie out to A, back from B), with these days very little difference in service standards.