It's not noticeably unusable.

Ummm, I wouldn't install it on a computer that you want to use for general computer usage. As a general rule, I advise people to wait until at least the .1 release, if not the .2 release for each new version. So using the beta is definitely a no-no, unless you actually need the beta, for testing your application against.

Otherwise you can find pretty big things not working - printing, might not work on one release, or bluetooth preferences might not open on another, or Airplay video streaming might not work. You may have graphics glitches, or a run away process that kills battery life. It's simply not worth the pain if you don\t really need it, but if you do really need it, the pain is bareable if you grit your teeth.

Just don't try to compile universal C binaries and you'll be fine

Unfortunately, it's a bug in CoreText, not WebKit. Either way, both frameworks are at the OS level.

Safari uses the OS-provided WebKit framework. An App Store update of Safari would only be able to change application-level features, not WebKit bugs.

Security relevant crash bugs are normally back ported. In any case, TextKit is just a wrapper of CoreText (like you said) for easier use within AppKit/UIKit — all the text handling is still done by CoreText.