I integrated CII into Easy-ISLisp without much hassle. Looking back I think it was worthwhile. My main motivation was to regain the more readable & safer string processing that C++ provided, without all the accompanying complexity. However, I found other features useful too.
First, I did the same refactoring as for OpenCOMAL:
then I went a bit further:
There are more parts of CII that are potentially useful, but I think this is good to start with. In particular, this refactoring paid for itself when with the simpler string processing code I was able to figure out how to reduce RAM usage (although more could be done here).
To conclude, I'd recommend using CII (with plain C) for future *low-level* projects. Obviously I'd prefer ISLisp for the majority of projects that don't require low-level coding.