I used study buddy and found it useful. The general rule of thumb is, go for your written when you get 90% or more on practice tests. I used the Sportys dvds, and I those combined with the Jeppeson textbook was enough, and I got a 93. The bad news is, after private pilot there are no more free study buddies. I use Gleim testbank, the written and the dvd bank, combined with ASA lectures on dvd. I am a slow learner, so I always take months to prepare for a written and use lots of study materials to help make sure I know what I am doing. I got 96 on the FOI which is my best so far and I used ASA dvds & manual plus the Gleim dvd testbank. When I got over 90% consistently then I took it. I have yet to take the CFI written, but am about 60% done studying now. It's hard studying for stuff like that when you have two jobs but I use my lunch hours. I have been toting the CFI manual around with me for months.

Private pilot is hard due to being new, but everyone agrees the instrument written is harder. I got 86 on it and was disappointed but had forgot to bring an E6B which cost me several questions. I just thought, you don't need an E6B for this test. " I was wrong"... as Ron White likes to say.