I just discovered MPS, downloaded it and tried the tutorial.
My overall impression: very good work, I'm eager to put it to work somehow. As always from Jetbrains: high usability standard,a thorough piece of software even in the beta phase.
The only (as of now ;-)) thing that bugs me is the fact that there is no textual copy/paste support in the MPS editors. I notices a related bug about this issue and was tempted to start a discussion in the bug but realized that this forum is a better place for it.
I realize that the text I see in the editor is just a representation of the data in the under laying xml file. But it is the mental model a human has, the xml representation is not suitable for human consumption (at least for most ;-)). So the way we want to communicate about expressions in a language is preferably the text as shown in the MPS editor. We like to email that, put it in a document or a presentation. I noticed that I can copy the text but I can not paste the copied (and possibly changed) text back into the editor. So if I emailed a snippit to a friend and he proposes some changes I can not paste his answer-snippit back into the editor, I need to visually find the changes and key in the changes myself. That is too tedious.
Another example is the tutorial itself. I had to construct the java code myself although I usually copy/paste code from tutorials, whcih is a lot faster.
Yet another example is resolving conflicts in a SCCS. I did not try this out, but I guess the only way to resolve conflicts is to manually merge the xml files. (Correct me if wrong.) Xml is not my mental model and therefore it is cumbersome to merge conflicts.
Bottom line: I would strongly urge for textual paste functionality in MPS editors (not just java templates). This would imensly increase the usability of the system.