Standing issues regarding it being OpenSource? Comparison on R vs. commercial products such as SAS og STATA? Introductory material to learn how to use R?
As mentioned by others there are many sources (several of them free) for learning about R. However, R has currently far too many different uses and thus you should be more specific on your goals to get better suggestions. Is your goal to learn the language or its use on a particular field/domain? What is the depth of knowledge you want to acquire in R? Without that sort of information it is hard to give you good advice.