What is the success rate for Invisalign Express in your experience?

I use Invisalign Express when I think 20 wks of treatment  will correct the minor crowding. That being said , It takes significant experience using Invisalign AND realistic goals/ expectations to make sure the technique meets the expectations .  Sometimes more trays ( weeks ) are required in cases that the virtual treatment is not achieved  and the expectation of the patient either change or have not been met. Extending treatment  ( more trays) time is possible but often times requires adjusting the fees. 

One nice thing about Clear Aligners is that I have the ability to view the before-and-after movie.  Many times, after viewing the short version (Express) for treatment, the results are unacceptable to me. So, I have the lab lengthen the treatment time to achieve the objectives.  So, in truth, neither the orthodontist nor the patient has control over treatment length.  What is the controlling factor?  It is the condition that the patient brings to the table.