There are a number of Indoor Free Flight classes that I have participated in: Embryo Endurance, Peanut Scale and Bostonian. Each offers its specific challenges.
By far, the most challenging aspect of model aviation, Indoor F/F will raise your level of frustration that you rarely experience in any hobby. However, it became the area of modeling that I learned the most about aeronautics. Once you get to the stage where everything starts to work and stops crashing, you begin to see the relationships in your airplanes that were hidden from you for so long. Free flight is that aspect of modeling where the control you have is predetermined and built into each airplane. The “Free” is where you release your plane to the air and hope that the controls that you set in the airplane are the correct ones. You do have a choice in how your airplane flies by understanding how the forces and relationships that are present act on your design.
You are responsible for how you trim your airplane. You do have control but it's a different kind of control. That control varies with the environment that you fly your planes in. In general there are two followings of free flight operation and that is either, Indoor, where the environmental conditions are highly controlled or Outdoor where the airplane is at the mercy of winds and changes of air currents. Each discipline comes with it's own set of challenges that have to be respected for top-notch model performance.
I worked at the Indoor Embryo class for quite a long time and published an Embryo design called Sharkies Machine, available here as a plan. I've also built several Prairie Birds. Peanut Scale is fascinating and takes your knowledge of flight to a much higher level with as many frustrations but plenty of real rewards. Bostonian is a fun class of models and takes the skills built in Embryo and applies them to a little broader parameters and this makes the models a little more fun.
I also participated in Outdoor Free Flight concentrating on P30 class, Coupe d'Hiver (F1G) international class and also Hand Launched Gliders. Hand launched Glider are a real workout and you will find muscles that you never knew you had operating these fast throwing airplanes. The Outdoor Free Flight classes are presented in their own areas on the web site.