Consistent? jQuery UI is one of the (not the, just high on the list of) least consistent and least flexible javascript tools I use daily. I have to hack around behaviors and inject buttons into areas that aren't blessed to have them all the time, and it's a royal pain because there are zero tools to help you and doing anything not explicitly supported (and some that are) often causes problems further down the rabbit hole. You've got to dive through div.ui-something > li.ui-another span.ul-cryptic div.ui-some-long-name just to get to the container, and God help you if you put something slightly too large in.
I hope they're at least more consistent than jqGrid. I've been using it heavily lately, and hacking on extra features, and holy cow is that an inconsistent mess. It's horrendous. It works quite well as long as you follow the examples, but try to do anything else and have fun! Out-of-date documentation, inconsistent interfaces, and just-plain-incorrect advice are absolutely everywhere.
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None of this is to say I don't think they can make a good one. I eagerly await it, and will give it all kinds of hell when it comes out to see what it can do. I just fear the API.