I use it, but generally only at work on a local network, where the performance is fine. My most common use is to run R on a big server but still have plots pop up on my laptop. There are other ways to do this, e.g. one of the IDEs for R (RStudio) apparently has some infrastructure that would let me run R on my laptop and then farm out the computations remotely. But I find just ssh -X the easiest for now.