I have been using evcxr for the last 2-3 weeks and I'm loving it. There are two things that bother me: (a) fancy UI components seem to only work with Python and C++ kernel (not theoretically, but with available packages), (b) while you can redefine values and functions when you're iterating, you cannot do so with structs (yes, big structs go to a local package that I depend on, but sometimes I have ad-hoc utility structs).