I've been using Radio to handle my blogging needs for the past year or so. It's pretty cool and I like how it works overall. The thing is, I really don't have the patience right now to dig into it and learn how to do things most of the time.
Also, I keep using Movable Type for lots of smaller projects. Each time I use it I'm impressed more by how well it works. It's definitely easier to jump into and play around with. It also has plugins. Bits of code you can easily drop in that extend how it works easily.
I read someplace that people have been moving away from Radio a little bit. At first I was wondering why, but as I've thought about it more I think I do know why. It just doesn't seem to be moving along fast enough. It does get some cool new features, but it's missing ones I want to use (trackback). Also, while there is a servre part for radio you can use, it kind of feels like a kludge to me. And, I'd have to move my web site to a whole new server.
I have to think about it a bit more, but I think Moveable Type may be in my future. Unless I can come up with some really good reasons to stick with Radio.