So at the beginning of this summer, a group of NYU CS students started on a project to build a decentralized social network, and then made waves when they raised over $200,000 on kickstarter, a crowdsourced funding website. They then proceeded to disappear into a cave for the entire summer, which killed the buzz around Diaspora pretty effectively. Then they put the project up on Github, and people immediately jumped all over them for security flaws. (Personally, I would expect to find security holes about that magnitude for a project this young. You fix them, and you move on.)
If I had to give my opinion of the project, it's somewhere around "cautious optimism". I'm not a Ruby or a Rails fan, but there are worse languages/frameworks they could have used. I think they're striking a reasonable balance between developing in secret and developing in public. On the one hand, they promised to make everything 100% open source, but on the other hand, the open source development model is pathologically incapable of making design decisions, and for the initial stage of a project you're making nothing but. I definitely like that they're piggybacking on existing protocols.
Apparently, they were inspired by Eben Moglen's idea of a "freedom box", which makes me sort of nervous, actually. Nervous, because the idea is good in principle, but completely unworkable and sort of silly in practice. Yes, it would be useful if we all had physical control of our own social media profile, but this has tremendous implications for the reliability of the network as a whole - if my Internet connection goes down, to what extent do I disappear from the web? And, of course, I'm glossing over all the real difficulties with hosting a website on a residential Internet connection. Quite simply, our infrastructure isn't up to the job, and I don't expect that to ever change. So, I kind of hope that the Diaspora devs aren't going to waste too much time on this particular use case.
There are also a ton of fundamentally hard problems that they are going to run into, and while I remain optimistic that they're thinking about them, we won't really know how they handle them until the software is in a more complete state. For example: how do you handle security updates in a worldwide distributed system? There are already a ton of insecure Diaspora instances running around in the wild, that people brought up as soon as the code landed on Github, and the problem is going to get worse unless they do something about it.
Overall, I have high hopes for Diaspora, but it's simply too early to make a call about the project. I'm expecting it to advance rapidly, though, and we may be looking at a 1.0 release within a year. Whether or not it's a "Facebook-killer", like people want it to be, it has a lot of potential to be a useful tool.