It's easy to get started with as you can use all existing DTOs (Data Transfer Objects) or write your own in minutes without needing any additional dependencies like WCF! The documentation could be better though - sometimes it seems that there are too many options when using some features which makes understanding what each does difficult at times but I guess this means they're trying out new things so will improve over time? Also not having an OAuth provider included isn't really great either since most providers have been abandoned by Microsoft now anyway..
We had several large projects where we needed RESTful Web APIs written quickly & easily from scratch rather than just writing yet another ORM/ORMService layer etc., hence why StackExchange was used originally instead because everyone loves them :).