Use BOTH!
You should ideally use both 1Password (or your chosen app of choice) AND iCloud Keychain. As if one fails, you have the backup to rely on.
There are some issues here:
1) iCK Utility apps?
Will iCK have apps for BOTH Apple platforms, so one can access them to clearly see the data and make changes easily? None so far in iOS 7 (though the website does say "coming soon", to be fair).
2) No debit/credit card CVV's stored?
On the WWDC keynote, Craig specifically said "we don't want to keep those credit card 3-digit CVV numbers though, that would be insecure!" (or words to that effect). Now that is REALLY stupid, as if I'm online shopping, I'd obviously want iCK to enter ALL my details including the CVV number. I certainly don't want to have to get my wallet out (it may be a card I don't carry, for example), as it would completely defeat the goal of it storing all my card details in the first place?! Other password apps don't have this issue.
3) Password site refusal.
Some sites do not let the browser store your password (as presumably iCK would rely on). E.g. my iPhone 5 is on iOS 7, I used it to log-on to the Apple Store site to check an order today using Safari, and while it remembered my Apple ID, it specifically gave a pop-up advising "this site does not let us store your password". Oh really, great Apple! And that was Apple's OWN SODDING WEBSITE?!
4) Cross-platform.
iCK is not going to be cross-platform, the end. So if you want access on another non-Apple computer (without having to manually type *^F)&OtgtdsdOs(*&(t(&(&•CIYvOUTkpmCVH^&)*(P&{I}_U)*(R from say, your iPhone!) you're straight out of luck. Again, third-party apps have that benefit. Unless they give users access at iCloud.com, so users could log-on in any browser to grab a password, but it's almost guaranteed they won't be offering this, just as they refuse to offer Bookmarks to be accessed that way, unfortunately.
5) What can you store in iCK?
Third-party apps let you store license codes, bank account details, secure notes, and a whole lot more items of data you want secure. iCK hasn't been promised to provide any of that, so will likely be very basic.
Obviously we'll have to see. But even if it's better than promised, I'll more than likely keep a third-party app as a backup anyway.
(BTW, for those who may want to try it, 1Password 4 for OS X has just been released, and looks to be on sale for 20% off – a bit stingy on the discount there, IMO they really should have given more like 50% off as they did before to allow previous owners of version 3 to upgrade for a reasonable price, but never mind!)