I've now received four Birchbox shipments and am very happy so far with the service.
I reviewed it initially in a post back in February. My favorite part about the service, besides getting an introduction to product brands that I wouldn't otherwise have access to or be exposed to, is that I like the tailoring to your profile. If you have combination skin, the skincare products are targeted for those skin concerns. If you like a particular type of perfume base, these are taken into consideration when perfume samples are sent your way.
This personalization is not something that's available from Sephora's online purchase samples, unfortunately. At Sephora's online checkout, you get whatever samples they have available at the time of your purchase.
A writer on the New York Times website (Birchbox Aims to Simplify the Business of Beauty by Jenna Wortham) wrote a piece based on Birchbox's April box, and focuses on the business perspective behind the service.
My only qualm thus far is that Birchbox themed its April box around eco-friendliness, yet the Zoya nail color samples came in a plastic envelope on plastic nail tips. I doubt these are eco-friendly corn-derived plastic?