It's a REST API in a server that accesses Ethereum. It's not a smart contract running directly on Ethereum. Rather it accesses smart contracts which themselves are 'on' Ethereum.
Not yet. As other commenters have pointed out there's a trust issue some might have for submitting transactions via it so we plan on open sourcing soon to deal with that.
Thanks! The private key concern is sensible. Our plan is to open source the code and make the hosted version auditable in real time to prove the code being run is 'honest'. Bit of work to do on this, still. I'll post an update when we get there.
Yeah BlockCypher have made some useful tools. Especially for Bitcoin. We're going to focus just on Ethereum because there's a lot of developer tools/infrastructure missing right now. Think infrastructure as a service, webhooks, data transport etc.
In the case of ETHEREST - it's just about being able to grab a contract address and call any method on it quickly and easily. Having to hook into a a node & provide an ABI gets frustrating really quickly, especially when you're testing.
On stability - it's definitely an MVP right now. Our plan is to open source the innards soon to a) make submitting transactions trustless and b) share some of the innovations we've made to make this scale well.