Nice work! This seems really slick :)

How has your experience of building with IOTA been? Is there a good SDK / documentation somewhere? I've just started digging into Ethereum and so far I've been a bit disappointed by the standard of the tooling... :/

Right now IOTA works as a content storage and distribution "database" regarding to what we are building, so it's more simple than Ethereum smart contracts and their own language. The API SDK's are all there and are working great for us!

Does the SDK call out to some server running a node somewhere, or does it actually implement a client side IOTA node in JS?

To check the data authenticity it's connecting to a node server for the data synchronisation, the data encryption happens on the client side. Client side nodes are on IOTAs roadmap as it's a technical challenge to synchronise with the network by just storing a small chunk of it.

Who runs the server?

What incentive do they have to expend resources to commit data to the ledger?

How can you trust that they have committed what you asked them to?

why is fog.sh storage better than dropbox?

It's not because of Apples and Oranges :)