I just installed it today, and I gotta admit - it's impressive! I'm logged into both Facebook and Twitter through it, and I can add other feeds, plus all my Firefox bookmarks automatically imported (as did my saved passwords, which saves a lot of time). The nice thing is that it uses less memory to do it all.
The Official Intro Video:
RockMelt - Your Browser. Re-imagined. Connect for an invitation.
Interview with the founders of RockMelt:
Pretty full review:
Only thing so far that I'm not a fan of is that right now I appear to be stuck with ads, and of course some of my favorite Firefox add-ons aren't there either. But as the founders mentioned in the second video, apparently you can import Google Chrome extensions into RockMelt, and some of the Firefox Add-ons I like most are also Chrome extensions, so, hey, this just might work out!