Well, I've converted over from pM to WP1.0 for a handfull of days. I can't say that there was anything "wrong" about their software - I enjoyed using it. For me, after playing around with WP for a while before making the switch, my decision points were:
- Open source. I prefer the model; I prefer not being restricted when I search for help (some pM materials are covered by the Pro license). Not to say that pMs free license isn't generous.
- Multiple blogs.
- I was having trouble with the TrackBack implementation in pM. It was working about 50%. WP has been fine.
There were some other features in addition.