Pretty slick looking, will still need some development to make use more intuitive. Like there's no clear sign that I just click back in the main area to return to focus.
Ah, but in this implementation, clicking on the new content doesn't make it go away. It's really not intuitive for a first time user (like me for instance).
We actually used this effect inside of Twiddla for a while, with universally bad user response. Ditched it in favor of boring lightbox-style pseudo popups.
I just tried out Twiddla, it's really cool, congrats.
As per this slide-in UI, I'm going to change my mind here and say you're right. It really is kind of pointless... it's the same thing as a lightbox, only vertical, and more disruptive and disorienting.
I do think there could be some use for it... maybe someone will make a twitter app that uses it.. twitslide perhaps.. and we can all be amazed that they did it in x hours just as a side project.
I can't wait to see where it goes though.