Are Buddhists happier people? The First Noble Truth says that cyclic existence is Dukkha. I'm not in a position to expound on the meaning of the word Dukkha, but I think everyone can agree it does not mean "happines". 

Still, I think many Buddhists would say they are more happy in their lives than non Buddhists. 

Would it be fair to claim that Buddhists, if they practice well,  can expect to be more content and generally more happy in their lives? And if so, is this because of Buddhism or is it more generally because they have a spiritual outlook on life?
If I can try to generalize, I would venture to guess that people who come to Buddhism on their own (as opposed to being born into the religion) -- are probably less happy than average, which is what pushes them to seek the way out to begin with.

The reference to suffering in the First Noble Truth is not meant as the final conclusion the Buddhists are supposed to live with, but rather as a starting point for one's spiritual journey. We are seeking an end to the painful wrongness: issues, troubles, hopeless social situation, messed up personal world etc. However, we don't believe in solving it by doing more of the same. We are tired of Samsara, tired of the rat race, we had enough of it. Buddhism promises an alternative to the rat race, this is why we like it, this is what we believe in.

That said, it's not that happiness simply comes as a passive side-effect of being a good Buddhist. Rather, deliberately cultivating happiness that does not depend on external circumstances is one of the core practices in Buddhism!

In Theravada, happiness is cultivated at the end of samatha stage, as the First/Second Jhana. In Vajrayana, this is practiced as Generation-Stage Meditation. In Mahayana, we achieve happiness by forgetting about our own problems as we take care of others.

All this is to say that happiness is not a second-class concept in Buddhism, far from it. In fact, in (Tibetan) Mahayana, one's capacity for lighthearted humor, affirming joyfulness and authentic warmth; one's capacity to be happy and to make others happy -- in the face of the most adverse circumstances -- is considered the most reliable indicator of one's spiritual progress.

So yah, it should absolutely be a fair claim to make, that Buddhists who practice well are happier than average, not because Buddhism is so awesome but because being happy is one of the main things we practice!