Easily accessible. Easy to learn, since it's very well documented and has a lot of discussion forums. Also, the availability of source files, makes it easier to actually realize and appreciate its working and gives sense of control over it and helps in understanding the underlying computations and physics, rather than working on a "black box" which is the case with most of other tools of comparable capability
I Started using OpenFOAM last semester in Feb, 2013. In the start, it took a while to get used to all the file system but later it became normal.Customization of problem's physics is very easy unlike other packages whereediting is restricted after a level.
I had never been introduced to OpenFOAM until last semester. So initially it took me some time to understand how to go about it, but once I got a hang of it, I could work well with it. The video tutorials were of great help.
I used OpenFOAM for my class assignment for "Turbulence and Combustion" in Spring 2012 at IITB, in which I asked the students to solve for turbulent channel flow at high Reynolds number using RANS (Reynolds Averaged Navier Stokes) models and compare the output with Direct Numerical Simulation data. I referred students to the FOSSEE spoken tutorial website for learning the basics of OpenFOAM, which they found to be very useful. One of the big advantages of using OpenFOAM was that the students could simply download the software into their laptops for free, and I did not have to worry about giving them access to any licenses. Moreover, the students were able to learn the software fairly quickly, and got a great hands-on learning experience on using RANS models.