We use Parse for some apps and roll our own Rails backend too. Of all the MPaaS providers we've investigate, Parse seems to be getting things right: SDK's are solid & the documentation is up-to-date, 3rd party integration with Twitter / FB / S3 work great. It's a joy to get started.
The biggest problem we've seen so far are on caching, user models, analysis and emails:
- Local native caching on devices seems capricious, and we've often reverted to writing our own caching schemes on top of Parse (had to do the same for server-side, too, but maybe that's a given...)
- User data model is pretty basic, and although the ACL capabilities are nice, it's not easy to build (FB invite requests work, but outside of that framework you have to roll your own invite / accept / reject / exit logic)
- Custom emails on User model CRUD / password are supported, but not for any other user interaction (ie., weekly roundup emails, status updates, etc.)
- API introspection is tough, it's not easy to optimize (ok, minimize) API calls and some form of introspection & analysis would be helpful (eg., just when do the SDK libraries synchronize, and how often?)
All that being said, I don't think any of their competitors have done a better job on these topics and certainly Parse seems to be pulling away from the MPaaS pack.