So I tried Microsoft's CV & IBM's AlchemyVision on the same image (since they both have an online demo sandbox). The Microsoft just gave me back more labels and stronger sentiment figures for same labels. Hence narrowed it down to these 2.
Disclosure: I work at IBM Watson.
qq - Is the API stabalized, by which I mean will there be further changes/merges?
We're continually improving them (as everyone in computer vision is!) but for now one key feature of Watson Vision is the ability to train a custom classifier for your own data by just giving a bunch of example images. The service also provides general image tagging, text extraction(beta), face detection and a fixed list people for celebrity facial recognition.
We see this as a green-field area where rapid progress is being made across the board... I wouldn't count anyone out yet!