There's only 3 times in my life that I have actually teared up when watching a television show or movie (Ironically all of them while watching Stargate episodes) This was one of them.
David Hewlett's performance in this episode was truly inspired. Undoubtedly one of the greatest performances in the franchise''s history. He made you genuinely believe that the true Rodney McKay was slipping further and further away. He conveyed so well just how devastating it would be for a character like McKay who's entire life and self worth revolves around his intelligence to slowly lose it. From his body language to the slurred and warm tone to his words, it was truly a remarkable performance deserving of recognition. Brilliant performances also by Jewel Staite and Kate Hewlett in conveying their characters individual and shared grief over Rodney's situation. And yes while I will admit there are plot holes in the flooded-planet situation (ie. Why didn't the scientist notice the glacier was melting?) David's heartbreaking performance makes those holes seem irrelevant.