Hey Chris,
I've setup SNS for Rails apps before but not for pushing to mobile. I've used it as a PubSub system, email notifications and with Lambda for Slack notifications.
Mostly, I've used it to connect services running inside AWS garden and it's been good for that.
My experience with it has been ok. It's a bit of a pain to setup, but once you have the permissions and ARNs worked out, it's fine. When DynamoDB had issues a few months back, that really effected everything, including SNS, so we were "down" for that part of the app, luckily it wasn't a "business crucial" piece.
I haven't tested it at any real scale, so can't speak to that.
The hardest part was coordinating all the Roles and ARNs, after that it was pretty straight forward.
Would love to hear how your experience with SNS goes as I'm considering using it for mobile push.
Best,
Misha