Recently I migrated from Blogger and Wordpress to Posterous since I read lots of great things and saw many people migrating as well. Sometimes you shouldn't follow the crowd without realizing what you're getting into. Here's my problems with Posterous at its current state. Hopefully this will help educate you on some of the quirks you may face. 

1. No "Save as Draft." Are you kidding me? Where am I supposed to jot my ideas, compose my posts, and let them sit for later proofreading? Apparently Text Edit on my Mac like I'm doing now. This blows my mind. I guess they're taking a new approach to things with this "email a post and we'll blast it out," idea that leaves traditional bloggers out in the cold. Call me selfish, but I'm thinking for us people that use our editors to collect our thoughts and create posts, that feature should be included.
2. CSS. Many people that want a simple blog don't care about this, but for people that are remotely interested in customizing a site for them, access to the CSS is necessary. I don't want to have to manually override with my CSS all the time.
3. The layout/features of the post editor. Why hide things like tags, date, and Autopost options? That stuff is important. Since it's hidden, I almost always forget to go check/uncheck boxes and tag my posts. Where is the geotagging box? On the other hand, the media uploader is nice.
4. Search. It doesn't work…at all. End of story.
5. Visitor Stats. Not that any other service I've used has done a phenomenal job of this, but I'd like to have more traffic stats inside Posterous. Instead, they have made it easy to add Google Analytics, which is nice, but I want something easier.
6. Speed. I don't know if it's the large about of scripting that's going on for the various functions on the site or the 3-5 page redirects I get every time I navigate to a page on my site, but Posterous seems very slow.
7. Mobile Support.  Posterous boasts about their mobile theme being included and ready to go, but I'm not impressed. Lots to improve there.

I'll jump off my soap box now. Hopefully these things will be addressed in future releases, but until then, be mindful of its limitations before committing.

Stick around for "What's Right with Posterous."