I have only worked on one legacy C++ code base, and it was a student job, but I've enjoyed it a lot. Most of "modern C++" can be applied to an old code base step by step, all the way up from C with classes. As long as nobody decides to introduce the project's first "const" somewhere in a core class... :)

I hope you didn't take that to mean that I don't maintain my own code. But I can choose when to refactor and when to put up with it. C++ isn't a tool to use when you can't own the project.