I use the +4 plugs in my '89 560SEC and also an E150 Ford 5.0 V8. I like them fine in both applications, although I can't say there is a dramatic difference between the +4 and anything else.
I tried Splitfires once in yet another (non-MB) car (guess I'm a sucker for gimmicks), they were advertised to improve fuel mileage or money back. I took the latter.
If the +4s perform okay as advertised for 100k miles, then it will be worth the expense for me, as well as avoiding the bother of setting the gap.
I have experienced no fouling of the +4s.
All that said, that is just the experience in my car, Benzmac has seen a lot more spark plugs than I will see in my entire life!