Blackest Night was one of the first things I ever read in the DCU. It's a great story, provided you started with Green Lantern: Rebirth, because that's where it all begins. This story has been in motion since 2004, and has just now ended. It really is a great saga, encompassing 50+ issues, plus the Green Lantern Corps series and crossover in Blackest Night. In short. It's a lot.
The problem with Blackest Night is the uncertainty of it. I'm not sure exactly at what level of Armageddon this is. Sure, Nekron might be obliteration all life, but it doesn't seem like he has a very good reason or thesis statement or really a good plan to do so. It seems like he's just gotten fed up with things existing and wants to do something. He's like the Grinch who stole corpses (and then proceded to terrorize Whoville with a zombie army).
It is nowhere near the crisis level of Final Crisis in which the entire multiverse is dragged into oblivion by evil gods and vampiric monitors who leech of the bleed between dimensions. It pretty much involves time dissolving into a singularity in which nothing exists but Darkseid.
Blackest Night just feels like an aftermath in the wake of Final Crisis. Barely any time has passed since FC and Barry Allen has come back. I guess Barry's rebirth is the trigger here, because Nekron is rather pissed about it. To sum things up he's just angry and has zombies. I honestly still don't understand why he allowed resurrections in the first place if he's so anti-life.
I think the message here is that life is hard and but that if you want to truly live you have to fight, but that was the message of Final Crisis. Blackest Night is great for the art and the spectacular full-page displays, but it lacks any actual meaning. The last issue does have some very good points and is quite quotable, but overall it's more of a massive fanboy moment than anything.
There's nothing wrong with that, but Final Crisis and Grant Morrison do a better job of meaning and metaphysics than Geoff. Geoff Johns is a good writer and his whole GL series has been great, but when it comes to dialog he's better with quips and one-liners than moving emotional words.
In short, Blackest Night is great and an important part of the GL mythos, but if you don't care for Hal Jordan, or Geoff Johns, stay away. It's a fun read but a lot of the actual power (which there is) is hidden beneath layers of combat and zombies.