It's not one of my favourite series but I do enjoy it.
My main problems with COD have always been the perfectly accurate almost no recoil 10 bullet a second 2-3 hit kill guns that just introduce so many problems with lag, spam through walls and a variety of other luck based things. Also: camping, simply because so many of COD's base gameplay mechanics favor the defender over the attacker, camping becomes a large problem (along with the rapidly changing spawns).
It's still an enjoyable, varied and compelling shooter, but definately not worthy of the best selling entertainment product, ever.