I found it adequate to work around limitations of another system I was using. We were using a container coordinator thing (Convox) that couldn't attach EBS volumes, nor could it limit concurrent access to exactly one replica. So I used EFS which worked OK. I kept an eye on how we were using the burst credits, and picked a filesystem size that gave us enough IOPS. All in all, it worked fine (but I was perfectly happy to move off of it to EBS, of course).