Does this imply that you only "mix" your coin with people also trying to mix them in those 40 seconds?
i.e. it works much better within peak activity?
As the transactions are large and incur high network fees, paid by the operator, there is an advantage to the operator to have multiple participants in one transaction.
But for users themselves as long as the coins in the sharedcoin wallet are being consistently swapped with fresh coins (tainted by many sources) it doesn't necessarily matter if another user is directly involved in your transaction or not. In fact the server can provide better anonymity (see below).
Is it important to use whole increments (like 0.1 BTC) when using this? If so, you should enforce that in the user interface.
(if you can send arbitrary amounts, that's cool, I don't know how this works)
Only if you are mixing with other users who want to send different amounts. When mixing with the sharedcoin pool the server doesn't desire any particular output size so can deliberately add input and output combinations which mimic your own. Then when you include randomised fees into the mix it makes it almost impossible to determine the inputs used from the value of one output.
Cool! Does it allow other users not using blockchain.info to join a coinjoin transaction?
Yes there will be a browser extension available which you can input a private key.