DC's first experiment with a weekly "Big Event" comic (52) was a well-written and interesting series that could easily be read as a standalone comic without buying various spin-offs. Unfortunately, its sequel (Countdown to Final Crisis) is quite poor in comparison. The writing is bog-standard, the characters are flat, and (most annoying to my mind) the stories don't make sense unless (presumably) one were to have bought and read several other series at the same time. The overall big idea is that a race of super-powerful aliens called the Monitors are trying to kill the various time-travellers, dimension-hoppers, and other heroes & villians in the DC Universe that are messing up continuity. I know the writers want me to side with the victims, but I can't help rooting for the Monitors--kinda like rooting for Lucifer in Paradise Lost I suppose. I always find that my interest in super hero comics waxes and wanes every few years--an exciting storyline or series (like Marvel's Civil War) will draw me in, but then the realization dawns that nobody ever really dies and nothing ever really changes, and then I gravitate back toward creator-ran non-shared universe series like Preacher, Transmetropolitan, and Sandman where the author is able and willing to tell a complete story.
UPDATE (Feb. 16, 2009): Hmm, for some reason, when I re-read this and it's sequel, most of the things I complained about above didn't bother me nearly so much. I think that knowing the story's flaws in advance made them far less irritating. I still wouldn't call this a good read, but I probably wouldn't be so quick to label it as terrible.