Wow, slightly unexpected conclusion to episode 8. While I guessed that Oberyn would fight for Tyrion due to the massive exposition he gives on the matter when he first appears I didn't think the duel would end like it did.
In terms of television it was a great example of rising tension with a sudden reversal.
However overall in the story I do think it is a little strange to have a character appear, have loads of scenes and then disappear completely.