This plugin is the first one I install on every single WordPress project I do. It has become an essential component of my WordPress experience.
If you need to have you or your users post from the front end of your site, this is the ONLY plugin solution that works. Few headaches, amazing support, and great (ever-improving) design. It's so flexible that you can do really simple stuff as well as deeply integrated tasks.
Plus, it's great if you just want to use it on the back-end to add extra fields to your posts/pages/etc.
If I could give this 6 stars, I would.