RStudio has some nice features that Statet lacks, but my experience with RStudio has been disappointing in terms of stability and proneness to crashing. They offer similar functionality and similar ease of use. I have used both Statet and RStudio extensively. For people who already use Eclipse for development work, Statet could be an attractive choice. You can also take advantage of other Eclipse plugins, such as the spell checker or support for Git.
Support for R Markdown has been added recently. Development is not as rapid as with RStudio, but new features do appear from time to time. It continues to be supported and typically updates with new versions of Eclipse. You can run an R console, debug a script, compose a document in LaTeX with R code chunks, and link the two through Sweave and the plugin. Stephen Wahlbrink has written a plugin for Eclipse, Statet, that offers similar functionality to RStudio within the popular Eclipse framework. Many R users enjoy working in RStudio, but it’s not your only option for reproducible research in R.