Implicit sharing vs. c++ refereces

[stampede]

I agree with last sentence. I was using stl containers for quite some time before I even saw the Qt hello world example, I loved Qt and the benefits of its container classes, but still I'm using const references to pass QStrings, QImages etc. even if I know that its not really needed. Thats ok if I had to work with "regular" container classes.
But I really like the simplicity that comes with copy-on-write. If I had a choice, I think I'd use implicit shared containers instead of regular ones.

adds code, memory and runtime overhead to your container classes

Code overhead ? You can write code that manages implicit shared data once, and then reuse it for most of your containers.
What do you mean with memory overhead, a pointer to data and integer counter ? I failed to see a big runtime overhead either.
In the end, it's your design decision. If you are implementing a set of classes only for yourself, you are free to do whatever you want. But if you want other programmers to use it, I guess more people will enjoy your code if its easy to use - and I think implicit sharing guarantees it.