Some background to this answer: The company I work for develops hardware for robotics researchers and educators, and we speak to many people about their specific needs in robotics hardware and software (from highschools to commercial companies) on a daily basis.
I'll try to keep this short, since I have quite a few opinions on the matter and also need to eat at some point. :)
1) Suitability for the task: This is what most people look at, but is only part of the story. First, ROS can be looked at as the evolution of the Player/Stage framework, which up until ROS was the best example of open-source software for this purpose. As well, from our experience, ROS may not be perfect, but it's as good or better (usually better) than anything else out there.
2) Documentation: The tutorials may not be complete, and a learning curve still remains, but it's better documented than anything else we've found that has similar capabilities, and there are a few companies (Willow Garage, ourselves, etc) who maintain documentation and APIs on the ROS site rather than on our own pages. For instance, it's in Clearpath's best interest to add as much documentation as we can to the ROS site, since it helps our clients help themselves. :)
3) Adoption: Research groups, commercial companies, and gov't organizations are beginning to fixate on ROS as the software architecture of choice for their work. I won't release the percentage of our clients who use ROS, but I can say that it's the majority. Notably, a large number of them were non-ROS users up until recently, but decided independently to switch over (which isn't a trivial task on its own).
4) Community: This began with "Adoption" above, but is reinforced by events like ROSCon and the establishment of the Open Source Robotics Foundation. The latter is a nonprofit dedicated to sustaining and improving open-source robotics software (ROS and Gazebo to start), and is the first organization of its kind to exist.
Hopefully the above provides you (and others) some insight!
@sam Well ! As of now, in comparison with other options (viz. MOOS, YARP, MORSE, ORCOS, URBI, Player/Stage, Microsoft Robotics Studio etc) ROS is clearly the leader. So, it should be worth your time to learn an absolutely awesome software platform for robotics.