I use Blojsom
and post with the Metaweblog API. This means that "categories" have two distinct meanings. On one level, categories are like sub-blogs. On the other, these same categories are a list of check boxes to tick for an entry. For Blojsom 2.x (and other Metaweblog API blogs, possibly), the sub-blogs and the check boxes are the same, but in order to have something in the right category, I have to post to the sub-blog. I have a feeling this makes precious little sense! I can give you a test account on my server, Daniel, if you wish.
With ME, I have to define a separate Weblog account by hand for each sub-blog category in Blojsom. On the other hand, when I create an account with ecto, the sub-blog categories are discovered and listed for me under the parent blog. If I add a category, I can just refresh my ecto account and get the new category listed. Other blogging clients (I have used Contribute 4) list each sub-blog category too. It's a minor annoyance - after all, it's not that hard to add the various categories - but it took me a long time to work out the first time I used ME.