Now up to version 0.08 (2), it still has some issues and is still under development, so feedback to the Wordpress/Nokia Developer Blog is much appreciated, but the version available in the Ovi Store does get the job done.
Of course, your usage will depend a lot on what you use your Wordpress blog for. If you're doing long articles with lots of multimedia (like my Eurovision blog) then this mobile client is best suited to checking stats and moderating comments. Although you can add pictures and videos from your handset, if the layout and appearance is important to you it's still trusting a lot to do so just from your mobile. On the other hand, if you have a blog with short posts and the occasional link (like my personal blog) then it's not going to be that difficult to update directly from your handset.
It's a good mobile client, and mobile is the key word here. It's not a portable version of Windows Live Writer, but it's far better than using the web browser and the HTML admin screens.