How does Mapbox stack up against what we've built? Well, they have way more people working on the problem, so there's going to be more support and more eyes on the problem. I didn't directly compare map tiles, that was a couple other people on my team. We decided to try moving forward with our own OSM-based tiles, so the conclusion I've come to is that it compares favorably.

How difficult would it be to build a competitor to Mapbox? Well, if all you care about are map tiles, it's no big deal. If you want to build a business around it, that's always work. You're going to need to build the tiles, have infrastructure to serve it, monitoring, staff to manage it, billing and charging infrastructure, etc...