The only problem that has been brought up by my colleagues and instructors is the fact that most (if not all) e-cigs are not yet regulated by any authority. Also, the FDA, WHO, and other national and global health organizations have not done proper testing and can't guarantee that these cigarettes are completely safe. Harmful, toxic, and carcinogenic chemicals have been found in cartridges from various manufacturers and nicotine has been known to be found in those claiming to be nicotine-free.
Personally, I see this being a great step forward for props and smoking on stage, but it really needs to be regulated, tested, and have support showing that we are not doing harm to our actors. In the mean time I would recommend that someone (props master, SM, Production manager, etc.) do the research on what exactly is in the brand of e-cig you are using and what (if any) testing has been done. Until a theatrical company produces a safe and legitimate e-cig for the stage I think we all need to be cautious and skeptical of all brands.