Quin makes valid points but "guaranteed potency and purity" applies to compounded products from good pharmacies too. I've tested vials from three different 503B facilities and all came back between 97-103% potency via Janoshik. That's within the same USP specifications as branded products.
The real value proposition of brand is the delivery device (prefilled pen is undeniably more convenient) and the clinical trial data backing specific dose titration schedules.