May I ask what this is for? I've had to learn R recently after working
with Ruby for a while and find myself thinking constantly "This would
be much simpler if I could only use Ruby!". As far as I've worked out
so far:

R's advantages:
Fast matrix library.
Inbuilt graphing.
Don't have to teach statisticians OO concepts.
Specialised.

R's disadvantages:
Procedural, with highly limited function support.
Slow loops.
Depends on blas (and as a result libfortran).
Specialised.

For most purposes, you could do better using Ruby with a good graphing
system (such as gnuplot). There are, of course exceptions, but I would
consider all options first.