Chad (who did the MT SB plugin) compares SB and the new RightFields plugin. It looks like RightFields, by the author of BigPAPI (which the SB plugin uses), provides an easy way of adding new fields to your MT installation. Related, but not identical, to what SB does.
SB is about being able to define structured content types (using Kimbro's MCD language) and have many people publish using your new type, then aggregate the posts later on, perhaps using something like PubSub to find them.
RightFields, on the other hand, is about being able to add a new field to your MT posts whenever you feel like it. You don't need to edit XML files to do this, so it's way easier to change stuff than with SB, and it seems like you can access individual fields with template tags, so it's much easier to customise the presentation. However, it doesn't seem to do machine-readable output (microformats, SB XML etc) - it's all about customising what you can do with your blogging tool.
So it seems like each offers different benefits to users. If you want to publish reviews, events, lists, audio/video or people/group showcases and leave the door open for people to aggregate them, Structured Blogging is what you want. Whereas if your goal is to seriously customise how your blog looks and change the sort of stuff you want to publish without having to edit any XML, RightFields is right for you.
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Paul Kedrosky thinks it will fail because people are lazy. The response to this is, as always, that people are already using the old SB plugin quite a lot. Why? Because it actually makes it easier to publish structured content. If you don't want to publish reviews, events, etc, SB is of no benefit to you, but as soon as you want to review a book, for example, if you click on 'write review' and then 'book', you'll be able to get most of the details filled in for you by just typing the title and clicking 'lookup'. So it's actually good for lazy people (err, lazy people who want to write book reviews :-)
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Joshua Porter says that there's not much benefit for users. He's missed the above point, though: if you want to publish reviews or somesuch, the plugins make it easier for you. The aggregation thing isn't currently such a big benefit for users, because this is the start of the bootstrap and nobody's done the review/event/whatever servers yet.