I started out using Radio Userland, but I can't recommend it. They might have improved it in the last year or so, but I found it to be buggy, poorly documented, and generally frustrating. Radio did come with some really cool tools and I liked the overall idea behind how it works, but I just found it to be poorly implemented.
I then moved on to Movable Type. After using Radio, MT felt like heaven. Lots of great plug-ins, very stable, and adequate documentation. Also, the MT support forums were populated with lots of people providing helpful answers.
After the MT 3 licensing debacle, I switched to WordPress. While this was a good decision for me based on the licensing issues, it was also a good decision with respect to usability. While WordPress doesn't have anywhere near as many plug-ins as does MT, it is way faster. From my perspective, the quantity and quality of built-in features is about even between the two tools.
If you decide on MT or WordPress, you might want to check out my step-by-step install guides:
Here's a quick comparison I wrote up about a year ago for a presentation. See which tool matches your goals and working style.
Movable Type – Power User. Desire high customizability, but want to benefit from large community of plug-in builders. Control of look and feel is very important.
Radio – Love outliners, proprietary scripting languages and object databases. Okay managing your blog from one machine. Want to get started quick. Want a built-in aggregator.
Blogger – Simple and free are most important qualities. You have a Google fetish.
Blosxom –Desire total control and have plenty of free time. Love the concept of Wiki webs. Love the concept of minimalism.
Other than the comment about plug-ins, you could replace Movable Type above with WordPress. And don't be intimidated by my mention of "Power User". While MT and WordPress give a power user almost as much control as he or she could ever want, they are both very well designed and can be easily used in their default configurations.
Best of luck with whatever you choose!