I grew nothing but bagseed up until my current grow. I am flowering BS plants as I write this. BS is a great way to learn some of the finer points of growing. Because the seeds didn't cost you any money, there is less pressure.
On the other hand, commercial seed has benefits as well. Genetics are solid. You can select strains exhibiting any quality you desire. I am currently vegging my first commercial grow (Lowryder 2). Buying commercial seeds allowed me to turn a corner in my grow operation. Now I garden at a different level. I am no longer a beginner.