Is it the simplicity of integration? The cost model? What's better about using a "gateway"?
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First, we are using Amazon S3, underneath Imgur.
What do we get? A clean/easy API yes, but they handle virus scanning, image re-sizing (profile images, etc.), backups (we also back our Imgur-hosted images up nightly), and they're large enough to be at a better price overall due to the deals they get with their host, CDN, etc.
It works - and it works well, I don't see any need to switch to another solution with many unknowns. Knowing what and who you're dealing with is worth something too.
I should also note we regularly work with Alan Schaaf who founded Imgur, and he's awesome. The profile image re-sizing we need for all the size squares we use? A few days turnaround on their side. Also, I'm working with them now on getting SSL enabled for our images.