When devotees of ISKCON preach, most often than not, it will be categorical in the sense that Krishna is the Supreme Personality of Godhead and everyone else is subservient to Krishna. In fact, sometimes, you will hear that the Gaudiya vaishnava siddhanta as Lord Caitanya is the supreme refuge for all living beings. In this, there seems to be an apparent implication that other paths perhaps are not correct or bonafide. For the untrained ear, this might sound sectarian and even fanatic hence offensive.
Our acharyas actually present Gaudiya siddhanta as the supreme occupation for mankind. In other words, they recognize other valid spiritual paths but simply focus their energy on the ultimate fruit of the soul. Of course, not all will be interested in the supreme fruit of the soul. We are all living entities with free will, so not many will be interested in the highest dharma. In fact only few will be interested in the highest dharma of love of God in a mood of separation as Srimati Radharani exhibited as Lord Caitanya. True love even of this world is the sweetest in separation than in meeting.
So the Gaudiya philosophy inspires the soul to love God in a mood of sweetness. This does not mean other paths are not bonafide. There are so many paths beginning from pure materialism to material religion to nihilism to sayujya mukhi to personalism. If you imagine these paths to be the different rungs of a ladder of dharma (material to spiritual), the Gaudiya vaishnava siddhanta of pure love in separation is the top rung after which there is nothing more for the soul to aspire. The spiritual journey of the soul is complete and hence ends in that topmost rung. Anyone aspiring for the highest fruit of life will ultimately take refuge in Lord Caitanya Mahaprabhu.
As I am writing this, perhaps it is already New Year 2015 in parts of the world. I wish everyone a Happy 2015! Of course there is no steady state happiness in material endeavors. Only pure love of God will nourish the soul and bleed unending happiness. So in that mood to seek unending happiness, I wish everyone a Happy 2015!