As long as MS doesnt fix the issue with MSE not updating signatures if the computer has been in standby at the scheduled update, its pretty much worthless because it will still claim everything is alright, despite it using outdated signatures.
Of course, if you dont use standby or are willing to manually modified the task scheduler entry to circumvent that issue and also dont mind a comparatively slow and resource eating scanner without a web-guard (more important than file scanners these days) and behavioral-scanner, then MSE is for you.
I think theres two things that MSE does good. Its really easy to use. Perfect for non-tech-savvy people (which makes the update issue that much worse, though). And its rootkit detection is indeed very good. But thats a thing I would expect from the company that should know the guts of their own OS better than any outsider should...