The API is intelligently designed, easy to understand, and well documented. Errors give reasonably informative messages, that make developing against it fairly easy.
The time I encountered a bug (related to using conditionals in templates), the support and development teams quickly fixed the problem and let me know as soon as it was deployed. I was able to switch to the new version and everything worked flawlessly.
Additionally, every time I have had a question about using anything in Lob the support team has always responded quickly. Review collected by and hosted on G2.com.