I shall never stop being butthurt about Oberyn dying, that character had so much more potential, the very cheap thrill of getting him killed in this scene is totally not worth the waste.
Nevertheless, his choreography was pretty damn cool, finally someone that swings a weapon faster than a cow. As for building up to this scene for two episodes I think they could've made a lot more from it. Kinda missed the nigh-impregnability of his inhumanely thick plate which required the use of sneaky tactics and accurate attacks by Oberyn, worked pretty well in the books. Also at the end bit it kinda gave of the impression that the Mountain was faking it and was reasonably okay, whereas in the books Oberyn really got close and Gregor killed him with just his arm strength while being speared to the floor. Just kinda strange to see someone wrestle himself on top of someone and speak normally while getting a pretty large spearhead straight through the body. They should have just made it longer.