If you have a blog, chances are you use WordPress in one form or another. Those new to blogging may be using the WordPress.com hosted variety while those with a little more experience will most likely have their own self hosted blog. If you fall into either of these categories, this iPhone App is definitely for you.
With Safari, accessing your blog's back end to add a post was very difficult and cumbersome. You could always email the posts to the blog itself, if setup correctly, but that was always more complicated then it was worth. This application takes all the hassle out of the entire experience and makes it very easy for you to grab your phone and start blogging away.
The graphical interface takes on the simplicity of the iPhone UI and WordPress UI without leaving it looking plain. You can easily configure the app to work with any blog that operates using WordPress just by pointing it to your site and entering in your username and password. If you're paranoid, simple set up a writer only account to use through the app.
You can do a lot with the WordPress iPhone app, including adding a title and tags to your post, setting it to post when you want, and even adding pictures in to it. But there are two drawbacks: You can't place your image in the post (it defaults to the bottom) and you can't delete drafts or posts you've made (which is more a safety feature then an issue). Overall it's a solid app for anyone looking to blog on the go or for those of you interested in giving blogging a shot for the first time.