When I was given assignments for SML, I was constantly running into x or y restriction whenever I tried something. Maybe my prof knew how to setup his assignment for maximum frustration using SML, and normal usage is not so bad (after you gain some skill in it), I don't know. But consensus from my class & some people on the net is that it shouldn't take hours to get a 10 line program working. Principle of least surprise is something SML does not follow. Also there's a lack of a support network & decent documentation for it compared to more popular languages out there. I'd rather use lisp than the traumatic experience SML was. At least the syntax isn't much to learn.