It was pretty big back in the day but it's never really brought up any more. And apart from a few notable exceptions (Clooney and Margulies) most of the cast haven't really done much since. It was most likely a case of it going on for too long (ER arguably, really went down in quality in 2001-2002), and there were too many medical show ripoffs.
shareI was a "casual watcher" of "ER" during its run (i.e., I saw some episodes and I missed some episodes...I didn't watch a single episode after Dr. Greene's dad died...I was pretty tired of the show by then and it kind of amazed me that this show went on as long as it did).
The POP Network has been airing old "ER" episodes. I watched a couple...on one hand, it was interesting to see the older earlier episodes again (which are my favorite). On the other hand, there was so much emotional drama within an episode that I was drained after watching them. Plus, knowing the fates of the characters kind of killed the "rewatchability factor" for me. I just didn't really enjoy seeing them again and won't be seeking them out to rewatch.