Double extend really does leave my lashes looking fuller and longer. It works best if you do not allow the primer to dry before applying the mascara. This is pretty much the opposite of most 2 step mascaras. If I allowed the primer to dry the mascara clumped...not a good look. The products washes off like they claim as well (in tubes) as long as you really soak your lashes with warm water to soften them first. Please do not rub or you may lose lashes with the tubes!
The con's as I said before I really try to avoid anything 2 steps, there are other brands on the market that have tubed mascara without 2 steps. Next due to formulation the wand really gets clumped up often due to the product drying on the wand. I regularly have to wash the wand to remove the buildup so as not to get the clumps on my lashes.
Overall I do like this mascara and I may even buy it again but probably only if it is on sale as I really don't use this one often.