For a project I've been building, I have used Clarif.ai, Google Vision API, Watson, and Imagga. From empirical tests, Watson has always provided poor, if not hilariously non-sensical, results. As an example, I use this image of a baby in a stroller (https://dl.dropboxusercontent.com/u/898689/IMG_2948.jpg). Here are the results in classification tags from the 4 services:
Clarifai: people, child, vehicle, woman, one, man, outdoors, emergency, accident, protest, adult, transportation system, carriage, portrait, wheelchair, safety, road, bike, leisure, wheel
Google: baby carriage, car seat, child, vehicle, diving equipment
Watson: performing, escalator, repairing, indoors, celebration, dancing, human, amusement arcade, bottle, baggage claim, group of people, appliance, tiger, people, big group, child, mixed color
Imagga: people portraits
These results obviously may vary depending on image subject, composition, etc., but I've basically dismissed Watson as a viable off-the-shelf visual recognition API. That being said, if you have a specific dataset of images that positively and negatively identify a concept, Watson's custom classifiers may be of interest although I haven't tried that.