I was a casual gamer prior to Halo. I enjoyed Goldeneye, but had never progressed beyond an N64. Halo was a multiplayer experience that had me knocking on the door of any of my friends who owned the game, because I needed a fix. Knowing that Halo 3 would eventually came out, I skipped the Xbox and bought a 360, just waiting for the next installment. I spent *hours* in Halo 2 with my friends just exploring the levels - trying to superbounce and escape the maps - in addition to playing multiplayer. I played the hell out of the H3 beta, and developed a backlog of "inferior" games that I just never got around to playing because I was too busy playing H3. I think ODST has the best story - one I've played solo and co-op more than the others - and firefight offered a nice change of pace and another means of stealing my gaming time from other games. Reach has actually been a means of keeping in touch with old friends, as I moved out of my home state Sept. 1st of last year. Being on a headset with old friends feels like being able to still hang out with them, and while it's not exactly the same, it's still better than losing touch.
tl;dr: the Halo series turned me into a gamer, has had me pass on other games to keep playing it, and has kept me in touch with people I can't see anymore. Plus, I love collectibles ^_^