Last time I checked (around 2 years ago), it was very unstable and did not run on many systems.

Looking into http://root.cern.ch/drupal/content/requested-cling-features: There is still "bug 11297: STL: vector of pair works if compiled, but not if interpreted" Which is something so basic that it has to work.

Apparently the bug report was created in 2005, and it's currently closed with "Won't Fix".

I'd really like a C++ REPL, but I am not hopeful for Cling.