For those who don't recognize the name, Clark ran one of the best Bitcoin exchange charts sites at bitcoin.clarkmoody.com (now part of ZeroBlock?)

So it's not really a polled API, for serious clients.

If one of these guys provided a more traditional fix/itch/ouch setup, they might get more traction with traders who are already active in other currencies/products.

Is there a way to get real time order book updates from the Kraken?

Another good API is Paymium's https://github.com/Paymium/api-documentation the most frustrating thing I find about exchanges API are race conditions when doing HFT, like submitting an order getting an ID, then asking about the ID to find it doesn't exist yet as the update hasn't be prorated yet.

That said, they're better than say BTC-e and OKCoin :)

Keeping a proper copy of the order book on the client side should be fairly straightforward.

Bitstamp's API makes it impossible to keep the full order book synced. They broadcast the top 20 rows over the socket, but for many situations that's not good enough.

Maybe one day and exchange will create a developer email list where they let people know when the API changes (Bitstamp has had an awful history of breaking the API without warning).