Though directed at the K-12 academic market, the Open Options site by the Northwest Educational Technology Consortium is still the best single resource that I’ve found for any organisation wanting a balanced perspective on using open source software. The site covers everything from total cost of ownership to the philosophy of open source. There are other sites that will give you more specific details (click on the "OpenSource" taxonomy link, under the heading of this post, for a number of references and comments).
In a recent evaluation of four systems that I conducted for a client, I was very impressed by the functionality of the open source LMS when compared to established vendors. Many open source systems now offer support services for a fee, providing additional assurance to users. When looking at open source or proprietary systems, you have to "compare apples with apples", and most importantly, understand your own needs before you make comparisons. Marketing experts tell us that most purchases are made for emotional reasons, so it’s best to establish your criteria before you go shopping.