My Maths has it's place, as there's no doubt it's useful for practice, but in my experience, it promotes an ethos of trying to do things in your head rather than doing workings on paper, so more likelihood of getting things wrong, and of course, you can keep trying with multiple chances to get it right.
That's the opposite of what happens in proper tests and exams when you have to show your workings and have to get it right (no chances to try again!).
A couple of years ago, my son had a maths teacher who set everything via mymaths, no text book use and no worksheets (thank goodness). In the end of year test, my son did poorly. The following year, a different teacher used a text book (without answers) and insisted on proper written workings, and then marked the homework himself. My son's maths improved greatly and he got over 95% at the end of year test!
I've no doubt MyMaths is good for teachers in that they don't have to mark homework etc (just take a report from the system), but certainly in my son's case, it didn't help his techniques. Used as extra practice alongside proper written work and marking, it's probably an excellent addition, but it's certainly not an alternative to proper written work.