I used one when I was a nanny a few years ago. The little guy was one of twins, he was 6 months old and could roll back to front but not front to back. He was a great sleeper, but at night he would roll himself onto his tummy and get frustrated because he couldn't roll back. So he had one, and it worked so well for him.
Once they're able to roll front to back, though, you don't need to use it anymore. And I'm sure there's some babies who they won't work for - the super active wriggle bots, like the twin's brother, but he didn't mind where he slept. As his mum said @He's fall asleep hanging from the rafters if we let him.

I'd say go for it.