I've been a Segment user, and advocate, from the beginning. It was highly recommended in the book Marketing for Developers.
One thing I'd like to see is a better free tier for developers starting new projects. Currently, you're limited to 1000 visitors per month.
Even indie projects can quickly run up monthly costs of $100+.
As a comparison, Mixpanel's free tier gives devs 20 million data points per month.