There are a couple interesting threads on EDUCAUSE's CIO Listserv that talk about using and Moving to Moodle. This one talks about the decision to choose a Learning Management System (LMS) and asks about Blackboard, Jenzabar and WebCT. Many of the responses talk about choosing Moodle. This one talks about the pains of switching from one LMS to another.
Changing from one system to another is a very expensive proposition, not because of the costs of licensing the software and its care and feeding, but in the re-training of thousands of users. In one post, someone says that if Blackboard doubled the $75K/year that it's costing this school now, he'd probably keep paying it, as the current price is half what he's currentlly paying Microsoft and provides similar fractions for lots of other very expensive stuff. That's all true, but the fact that once you've gone with a proprietary system you're forever beholden to their whims is what I get from this line of reasoning.