DYLAN MCNAMEE I like Simgua quite a bit. A few months ago I bought the educational version to teach myself system dynamics in order to do some volunteer teaching at my children's elementary school. The documentation is good - gets you going from scratch, and gives a decent introduction to system dynamics overall. The interface is a bit clunky - the graphical view is for display-only -- editing is not WYSWIG, but it works fine. Updates are coming regularly, and each one adds powerful analyses or features. The educational price is very good. Since I haven't used STELLA, I don't know how the features compare, but the full price of STELLA is over twice that of Simgua.
(Version 3.4)