We've been using 1.9.2 for client projects since the release of Rails 3. It's been a great move with very few code compatibility issues. 1.9.2 provides a very solid boost in performance over 1.8.7, especially when rendering ERB templates, which makes a surprising impact on user experience for complex views. I've seen 3-4x decreases in ERB rendering times just switching between 1.8.7 and 1.9.2.

This isn't about scaling the web tier, which is fairly trivial, it's about being able to bring a request that took 500ms to answer down to 200ms. That can have a very real and appreciable impact on user experience.