Safari's been able to remember your password for ages now (only the syncing part is new), but it will now also suggest a random password for you when you're prompted to create one. (This is good, because simple passwords are insecure.) Then Safari will save the random password in the keychain, so you never have to remember it. Safari can also remember your credit-card information and automatically fill it (well, most of it--it won't store your card's security code, which Apple says "is in accordance with industry practice" and I say is a silly practice) when you want to buy something.
I'm also a little concerned about the fact that, when my Mac is on, every password is available to anyone who uses my Mac. If you're concerned that other people will have access to your passwords and credit-card numbers (but not the credit card's security code!), you'll need to set your Mac to automatically lock when it goes to sleep or when the screen saver activates, and set a very low timeout before that happens. I think I'd prefer an option to have to enter a password to unlock my iCloud Keychain. You can set the Mac's normal keychain to lock after a period of inactivity, but the iCloud Keychain can't be set to auto-lock. That seems like an oversight to me. Is the iCloud Keychain so secure that it doesn't need its own separate password lock, or so insecure that it can't be trusted to hold your credit card's four-digit security code?
I can quibble with some of the details, but I have to say that I'm impressed with the Safari update for Mavericks. Shared Links is a good idea that I keep finding myself using, and iCloud Keychain adds a level of convenience (especially once it's paired with iOS 7) that's hard to beat. Plus, any tool that discourages the use of passwords like "password123" makes the world a better place.
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