It depends largely upon who is giving the talk and what it is about, really. I myself asked a question some time ago inspired by a TED talk by Lesley Hazleton on the life of Muhammad, asking if her book was worth picking up, and /u/shlin28 (who is probably absolutely bewildered as to why they've been tagged in this post 9 months later, but great answers deserve credit and recognition) wrote a fantastic response here.

The thing about TED Talks is they are intended to be both entertaining and informative. When you attend a lecture at a university as part of a History course, you aren't supposed to be going in there as a blank canvass looking for knowledge for its own sake - it is usually assumed that you have already familiarised yourself with a selection of key literature on the topic (or will do very quickly afterwards), and the content of the lecture is structured according to the content of the reading and the specific learning objectives of the course. Whilst lectures can and should be engaging, their purpose is to help you navigate your way through the literature and more make sure you know what you're talking about when it comes to writing an essay six weeks later.

A TED Talk though has all kinds of criteria attached to it. It has to be succinct, it has to be entertaining, it has to have an appeal to people without an apparent interest in the topic and it has to stand alone. That imposes limitations on how much justice it is possible to do to a topic (and usually it's hard enough to do justice to most topics even in an hour-long academic lecture, nevermind a TED talk). Whatever you say in a TED talk will inevitably be something of an over-simplification; something intended to make you want to go and learn more, rather than an authoritative lecture. In that sense, they're much like documentaries - most people want to learn about the past but they wouldn't much enjoy a long, boring 3 hour special where historians debate the minutiae of how a particular event in history has been interpreted.

As it happens though, most historical TED talks probably aren't given by historians. They're often given by other very smart and experienced people, but that doesn't mean they're necessarily as qualified as they probably should be to be speaking on the topics they are. Kevin Kelly for instance has given a few talks on technology. Whilst Kelly is probably very well qualified to talk about technology in general as the long-standing editor of Wired and probably knows a lot about the history of technology, he is not a historian of technology; you should not take claims as gospel or assume they reflect the wider consensus in scholarship. In contrast, a few years ago Neil MacGregor gave a talk on a particular archaeological artefact; as Director of the British Museum and former art historian at a university, it is fair to say he probably has a solid grasp of what he's talking about. Even so, the caveats about the limitations of a TED talk still apply.

The other challenge of TED talks is that you can't very easily check their sources, regardless of who is speaking. In an academic book, you would expect rigorous citation of all sources consulted. In a popular history book, you would hope (usually in vain) for a similar standard but at least expect a bibliography of content and some idea of where the primary source material comes from. In a lecture, you have confidence the lecturer can give you supporting sources for anything they say in their lecture even. Here on AskHistorians, it is expected that you be able to present material in defence of any argument you advance or point you make. It simply isn't possible to chase up source material from a TED talk in the same way, most of the time. That does make them rather problematic as sources - not in the least because they may sometimes be making arguments that aren't widely supported by other historians.

TED talks can be useful as a window into a topic, whether that's by way of introducing you to something new or challenging popular misconceptions. On the whole though, their worth is for the most part quite limited - they usually require a lot of over-simplification and it's difficult, not to mention time consuming, to chase up the material they're basing their talk on. Many who give such talks might not even be particularly well placed to do them the justice their topics deserve (much like Hazleton in my opening paragraph). If you're trying to determine how reliable or useful a TED talk is, the best thing you can do is chase up the author and see who they are, on what they base their authority and if they've written any books, essays or journal articles on the topic. Better yet, chase up the elements of the talk that interest you and see what historians have written about them.

Certainly, you should be very weary of assuming something is an undisputed truth simply because it was in a TED talk - and I would strongly discourage anyone who might try and use them as a citation when writing history, possibly with some rare exceptions (you might feel you have good cause to quote George Takei's talk on his experience of internment during WWII for example, but you certainly shouldn't quote him as if he were a historian). If someone is properly qualified and says something of worth in a TED talk, they've probably put it in writing or a proper lecture somewhere else, almost certainly with more attention to detail. That's very much true for both academic and 'popular' historians alike. Or should be, at least.