I started with WoK for a patron at my library because he was looking for a large, well-written book with great characters, and he loved it. I don't know if I would start everyone off with that book, though, but it was my first intro to Brandon's work and it worked really well for me. The vibrant characters were what kept me hooked and so I never felt like the book really dragged. The size of WoK can be daunting, though.
This patron has also finished WoR and is on the 2nd mistborn book now. I'm not sure how I'll tell him about the Cosmere, but I'm thinking of just giving him Arcanum Unbounded when the time comes and sending him on his way