Managed C++ was chock full of __keywords for .NET memory management - like Apple's ARC in the worst case, except there were no good cases. While it probably was a clean superset of C++, it was painfully hideous.
CLI/C++, by which Microsoft replaced Managed C++, added new operators and keywords without much regard for backwards compatibility (=without underscores).
The implication being that people value code readability over __backward __compatibility. I have no data on the popularity of Managed C++ vs CLI/C++ though. I was only a fanboy because CLI/C++ looks awesome and I generally enjoy Herb Sutter's work, but then got sucked into the Appleverse.
Just adding __ keywords is not the approach I described, though. I described adding __keywords, and adding a standard header that redefines them into non-underscore keywords. This avoids breaking old code (it won't include the header) while allowing new code to use nicer keywords, if it wishes.
Right - I was just trying to provide context :) If there is anything in the Managed C++ -> CLI/C++ transition that is better than the ISO C approach, it is maybe the contextual keywords in CLI/C++ - something that can't be achieved with #define.