I do agree with Mill that it is better to be dissatisfied than satisfied. I think that if you are someone that is not satisfied or has to struggle to achieve something then you are more motivated to accomplish your goal. Also, in the end, when you achieve you goal, you obtain more out of it than someone who is just satisfied in their life. If you had to fight for this and struggle to reach this place in your life them you will appreciate it more than someone who did not.
In regards to Mill's argument that he knows this is better since you have experienced both sides, I feel like it is lacking some in its explanation. I do agree that the dissatisfied individuals do know both sides, but I am not sure if that if the exact argument that we are going for here.