I personally think the series is overated. I've never really been able to get into the games. I've got the PC versions of 1 & 2, and just haven't been able to see what all the fuss is about. I notice they don't seem to have the high review scores on the PC but I would attribute that to the fact they come out 2 yrs after when all the hype has died down a bit.
I'm pre-dominantly a "complete campaign" mode; dable a little in multiplayer kinda gamer. The campaign modes in Halo feel really quite generic to me. Don't get me wrong, it can be fun, but I never really felt fully engrossed in the games.
I've played a little Halo 3 and whilst I throughly enjoyed the co-op, it still feels a little too generic. Its obviously got a strong (near-religious) fanbase, which is why a Halo title will always have good sales, but based on what I've played I'm not sure why.