The ease of use and how quickly I was able to get up and running once I had all the configuration done. There are some features that aren't as intuitive, but not hard to find if you're willing to do some research. It took me about 4 hours to set up my first server. If you know what you're doing this shouldn't be too much of an issue. We are using OpenSimSim to schedule meetings that are open to non-employee guests. This gives our clients another option for attending to their events without having to worry about staffing or keeping schedules in house.