Opinions abound on the web. What is the state of the current science regarding this theory and what is the best evidence? Is the theory gaining or losing traction? If it's losing what's the best of the alternatives? Perhaps there was more than one major cause. What changed peoples' minds either way pro or con?
No, it did not definitively single-handedly cause the KT mass extinction event. Around the same time, the Deccan Traps Large Igneous Province (India) was being emplaced. Flood volcanism has been associated with other mass extinctions, due to the impact on climate, sunlight at the surface, etc, of the output of huge volumes of sulphur-based gases.
The Chicxulub impact probably didn't hurt the extinction, but the large particulate material it distributed in the atmosphere would have sedimented out again entirely within a handful of years. Meanwhile, the Deccan Traps contain sediment layers intercalated between lava flows, some of which contain fossilised dinosaur eggs.
The reasons for mass extinctions (and whether they actually occur at all, as distinct identifiable events) remain poorly understood, but we're pretty sure the meteorite impact wasn't the only thing that did for the dinosaurs.
Though I agree with @kaberett that there is indeed more and more evidences that the Deccan volcanism was the main trigger of the K/Pg crisis, i wanted to add that there is a more nuanced hypothesis (that I heard about last week during a talk at EGU) according to which the Chicxulub impact resulting seismic response may be the trigger of one of the main stage of the Deccan eruptions (Stage II if i remember correctly). This idea is based on the seismic modeling of meteorite impact by Meschede et al. 2011.
Meschede, M. A., Myhrvold, C. L. and Tromp, J. (2011), Antipodal focusing of seismic waves due to large meteorite impacts on Earth. Geophysical Journal International, 187: 529–537.
Richards, M. et al. (2014), Possible Triggering of the Largest Deccan Eruptions by the Chicxulub Impact. Geophysical Research Abstracts, 16: EGU2014-3042.