I've watched every single episode of ER, and season 13 is definitely one of my favourite seasons.
The new cast addition of Tony Gates takes a little while to integrate himself in, but after a few episodes you'll wonder why he wasn't in sooner. Everyone puts in a fine performance this season, especially Goran Visnjic with the Ames storyline (excellent, if a little over-long), and Scott Grimes' best episode, #7 (Jigsaw) when he gives genuine insight to his character, and the way people perceive Doctor Morris. Laura Innes coasts, but this is fine as she isn't really given much to do, and Shane West to be honest might as well not be in the first 15 episodes, he contributes so little, but this is (I feel) more due to the writing than his acting, which is a shame because he was popular in season 11. However, the season finale is fantastic and he does a good job.
The episodes are well written, especially for a show in its 13th season, and they dont feel like rehashes of old episodes. The aforementioned 'Jigsaw' being a highlight, as well as 'Scoop and Run', 'Murmurs of the Heart' and the last three episodes. Parminder Nagra as a surgeon seems more fitting than an ER doctor, so this development is very welcome.
Overall, the season is brilliant, a definite purchase!