So I've been trying it for a couple of days now. It's good if you use more than one social media site: you can collect all your updates in one place, and, more usefully, you can post across your different platforms in one action. It's more flexible than those Twitterfeeds that so enrage people on LJ, because for each and every post, you can select which of, say, Facebook, Twitter, your blog, Linkedin or Flickr you would like to post to. You can tailor content to your audiences - what I say on Linkedin is very different to what I'd say on here, and it's rare that those two would overlap. It's been interesting seeing what I'd post on Twitter but not here, especially as I don't always understand exactly why.
You can post in two ways (three if you just update on the site): put a button in your toolbar, and then, whenever you want to share a link, you click and it allows you to pick an image and add your own text. Or, and I like this a lot, email. All you have to remember is the simple email address firstname.lastname@example.org (it recognises you from your email address) or, to select your sources, say email@example.com
What this last means, is that even folk who can't get their heads round blogs- say, elderly parents- can blog now, if they can email.
There are a few things I don't like. It hotlinks the images you post, possibly incurring wrath from their hosts, and meaning that your blog won't last with its picture links intact forever.
You can't control the privacy levels of your posts (beyond public or private), and for some reason which is probably to do with US time, it backdates LJ posts, which as we all know, means they may well not get read on a busy Friends' page. I can't find a place to adjust time- SURELY they haven't assumed everyone's American?
Anyway, in summary, I recommend it, and look forward to seeing your Posterous links soon! Let's just say, it's more immediately useful to me than Dreamwidth.