My experience, and now granted I haven't used JMP since v4 alpha, is that JMP is a lot easier to use than R. R is similar to S-Plus in that it is an interpreted language. JMP has a scripting language, but its first catch was that you could create graphs and analyses (and customizations for both) using a GUI. Its learning curve for doing the basic stuff is a lot better than R, and perhaps appeals to a wider audience.

R has some GUI projects in the works, but nothing out of beta (please correct me if I'm wrong) and certainly nothing so refined as JMP.