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For now 0x mainly deals with exchanging of ERC20 tokens which are Ethereum platform tokens.
Not exactly but Lightning network could be somewhat similar to this where in Bitcoin can be automatically swapped to any other coin and then application could be built around it.

It's designed for Ethereum's blockchain due to the massive influx of tokens that have been created on Ethereum's blockchain. This is their way of providing a platform so all of these tokens can establish some sort of organization to better appropriate the usage of the blockchain to avoid backlogs of transactions.

It's more like application would be using 0x protocol (apis) to exchange ERC20 tokens. So ideally you can even build your exchange using 0x protocol that can trade Ethereum-based tokens between known counterparties.
Yes, good projects like Augur, Aragon, Melon, Status are using 0x platform. There are several good projects but it's now really crazy to invest in almost each of them.

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