I have tried out both 1 and 2. My experience with them is amazing. Hope to repeat playing them if time allows.
I didn't play the first one, I simply grabbed one of the sequels from gamestop a while back and just started to play it.
I started out playing the very first Borderland game. At first, I got confused and I didn't know how to play it so I easily quit. But my brother taught me how to play it and I understood it well. I got very addicted to this game.
Yup! Although I played Borderlands 2 first before I bought and played the first game. I was hooked with Borderlands 2 and it would be an injustice if I do not make time to play the other Borderlands games.