'Casualty' is set in a British hospital called Holby City and the show follows the doctors and nurse's lives at work and off duty. I usually dislike romantic sub-plots in shows like this (I stopped watching 'The Bill' because of it) but in 'Casualty' I really like it. We get to know the characters as people with lives, not just doctors or nurses. The story-lines are interesting and gripping most of the time. And character story lines usually carry over many episodes.
It's not the most intelligent show ever written but it has a nice charm, and if you fall for it like me you'll be hooked!
I also love the shows symbol, 'Casual+y'. Very good!
But a word of warning for new viewers, sometimes I felt the show was a little boring at the beginning. I feel this is a show that really grows on you over time. It is very character based, so you need some time to get to grips with all of the staff. I stayed in there and love it now.