I've used R since 1.4 ~ (2001). Compared to other dynamic languages, I think it's harder than most. Why? Because there's a lot more semantics. There might be R one order of magnitude more keywords for R than for Perl, Ruby or Python (PHP doesn't count :) ). There are two object systems, none of them very intuitive. You need to learn functional programming. 1600 packages and counting means more functions and deep concepts to understand. And last, you need to understand the math of whatvever it is you are using. Forget doing point-and-click, black-box analyses on SPSS.

Downsides include poor programming tools/debugger and pass-by-value. Still, it's one of my favorite languages.