We use it at http://virtualstagingsolutions.com and rely heavily on file uploads. We started using it right when they launched (back when they were filepicker.io) and initially there were some bugs with resizing images. Lately everything has worked pretty well. Their support seems pretty responsive and I've only noticed 1 period of downtime within the last 6 months or so which completely made it impossible for our users to upload anything.

I'll admit that I'm not 100% comfortable using them because I'm really not sure where they are going in the future, by the looks of their pricing plans it seems like they would need huge scale to turn a profit and I just don't see them getting that scale at the moment.