Dinosaurs » Conclusion
There has been no settlement to the issue so far, and no clear one is foreseeable. Both sides claim to hold the majority of proponents in science; it seems that (greatly over- generalizing) many paleontologists lean towards the intrinsic side, while many astronomers and physicists favor the extrinsic side, and geologists are probably evenly split between the two.
All of the evidence cited for the extrinsic catastrophist side is claimed as evidence by the intrinsic gradualists for their side or against the opposite side- volcanoes could create the iridium layer, shocked quartz, soot, and impact ejecta; the makeup of the iridium layer is not uniform in all areas, so it could be meaningless; and so on.
The main problem with both hypotheses is the issue of the selectivity of the mass extinction; some organisms were wiped out, while others were unaffected. Can climate change really explain the differential selectivity of the K-T event? The lack of understanding of the physiology of dinosaurs makes the issue more complex; if they were endothermic, why did they not survive like birds and mammals? If they were ectothermic, why did small dinosaurs not survive like small reptiles?
Also, many studies have focused on the extinction of dinosaurs alone, and have forgotten about the more substantial marine ecosystem collapse. The fossil record suggests that some marine reptiles died out several million years prior to the K-T boundary.
Other major problems with the issue are that it is not easy to prove (test) causation (as noted before), and that most of the ages of the rocks that different evidence comes from are questionable.
The two main schools of thought are split fairly evenly among scientists familiar with them. Either an intrinsic or extrinsic cause for the extinction would have complex biotic effects on ecosystems which would look confusing in the fossil record. There could well have been different, even separate extinctions in the oceans and on land; the marine fossil record does support a slightly rapid decline, while the terrestrial record (especially in North America) strongly suggests a more gradual decline (but again, has a fragmentary fossil record). If an extraterrestrial impact occurred during a gradual decline, which might explain the seemingly contradictory evidence. If looking for an opinion from a paleontologist's point of view, it seems that the simplest explanation is that the climatic changes induced by the shifting continents and the regression of the continental seaways were the ultimate cause (at least in North America), but this has not been (and may not ever be) proven. There is much work to be done, and much value to this work -- understanding the K-T extinction would help us to understand mass extinctions in general, and might provide a glimpse into the fleeting, evanescent nature of our own mortality.