I haven't played the whole series.
I bought Allied Assault for PC because of all those Omaha Beach trailers. The game wasn't fantastic or anything, but it certainly wasn't bad. Had enough of a Saving Private Ryan feel to it at times to make it entertaining, despite it's complete lack of story to connect the missions.
I got Frontline as a gift for the GameCube. It felt like more of the same that I got with Allied Assault.
I bought Rising Sun thinking it'd be awesome, because the Pacific Theater is very rarely portrayed in games. It felt like more of the same.
I borrowed a copy of Airborne while I was working at an electronics store. It felt like more of the same, but slightly prettier.
I thought about buying Medal of Honor when it came out last year, but after having played Modern Warfare and Modern Warfare 2... it looked like... well... more of the same. Found out later from friends that it was, indeed, more of the same.
Granted, by saying this, I'm not saying I felt any of the ones I played were bad or anything. Just that they all felt kinda bland to me. No real hook to draw me in. And I like military history...