It's not that either series is out and out bad, it's that there were much better options out at the time of peak popularity for each series, and the gamers who had been around longer and played the better games were miffed that something so mediocre by comparison was constantly getting called the best game ever by people who had played little aside from that game. When the original Halo came out, for example, I found it slow, plodding, and plagued with awkward jumping mechanics, because the other games at the time were mostly arena shooters, in the vein of Quake and Unreal. Now that almost every game on the market is in some way a descendant of Halo, it actually feels fast paced and high mobility by comparison, and I've grown to like the series.
CoD is an odd duck; it's a lot of fun for splitscreen, which is why I kind of enjoy it, but the mechanics of the current games in the series are so broken that it's hard to understand why CoD is the current king of the hill, and not, say, Battlefield, TF2, or even Halo. I enjoy it on the level that I would enjoy versus on an old Mortal Kombat game -- and anyone who spent time on those broken excuses for games in their youth knows what I'm talking about -- but I can understand why so many people are upset at its rampant popularity.