My first impression of it so far is: oh, it's quite powerful. Lots of things that you can do but all around one thing: you create a subject, called a twine, and post links to pages on the web that have to do with this subject. You can subscribe to twines, post to whatever twines you want, add comments to sites other people post, and that's mostly it. Posting can be through those funny links on your browser or you can send an email with the link to a special address @post.twine.com.
I'll keep on analyzing it and see how I could use that to reorganize my collection of bookmarks in a way that they might become more discoverable. Currently I'm mostly using del.icio.us, and there are a few things that bother me about it:
1) The big bucket of bookmarks with tags - I have work bookmarks (that point to Amazon internal tools) and external bookmarks. I would love to be able to partition them better. I like that I can share things I found at home with work and vice-versa, but I can't say I like when I'm at home and get "spammed" by work links when I'm searching for something.
2) The integration with Firefox for special keywords and things like that is not that good. Sometimes I set it in one computer and the other one doesn't really understand it. Then I create a new one o the other computer and suddenly I start getting warnings that there is a keyword conflict between the current computer and del.icio.us. Annoying.
3) "That's it": it's all what del.icio.us does for me: it organizes my links. It doesn't really offer much of a discoverability path. Yes, I can click on my keywords and find what other people have tagged with the same keywords and even read new things that come to those keywords as an RSS feed, but there is no de-duping, no discussion, just a stream of mostly noise.
Certainly Twine doesn't currently offer any integration with a browser, so I'll probably still use del.icio.us for some time, but if I can start transfering a minimum amount of information there and see how it turns out.