I have never actually played any of the series, but I know that they are really good games. My friend talks about it all the time, and it sounds amazing.
I've played The Orange Box, the Half Life series has amazing graphics, awesome gameplay, and the physics are accurate too(even the first one if you compare it with other games of the time). [My friend says he dosent like Half Life just coz of the name, he says it only means you are half alive. lol]