Starts off a masterpiece, has like a dozen mediocre episode, then finishes strong
A teenager finds a death note lying around. Is it for real? If so, how will it be used to change the world?
Death Note is a deep series mainly because it lets the viewer decide who's right; is it a battle of good vs. evil? Crazy vs. good? Crazy vs. crazy? Evil vs. evil? Are you on L's side or Light's side? Or perhaps you'll take a third option?
Either way, this show will be a great ride. The plot is so amazingly well-written, that when an action scene occurs most audience members will go "Hey! Enough with this action! Get back to the dialogue!"
Admittably, the last 1/3 of the show is very weak in comparison to the masterpiece known as episodes 1-26, but the ending is good enough to make you want to watch it over and over again. The last 1/3 is also possibly the part of the show where the most character development occurs (With Light's morals decaying to the point he even considers killing off lazy people and Mello getting over his pride to work with a certain enemy of his, for example). Yet it's still a lot worse than the rest of the series. Perhaps this is because it lacks L?
Oh well. Even without the best character it still manages to hook you it; watching 1 episode at a time is impossible, so good thing it only has 36 episodes instead of 360.
What impresses me about Death Note most is how the author managed to keep all the character's thoughts and knowledge consistent; there are never any bizaare inconsistincies like L suddenly knowing that Light hid a TV inside a bag of chips; the author manages to keep track of which facts the characters know and/or don't know. I imagine the guy who wrote it is also the kind of guy who lets his brain relax by solving Rubix cubes without taking off the stickers and rearranging them.
The manga for the most part is better (For example, there's one instance where Near figures out the identity of a certain character. In the manga, he lays out his thought process, but in the anime it seems more like he took a lucky guess.) so it'd probably be best if you read it.
Overall, I'd like to give Death Note a 10/10, after all it is one of my top 5 favorite shows, but thanks to the last 1/3 of the show screwing everything up, I can't give it more than an... well actually the ending is pretty powerful. No matter which side you're on, it'll make you feel for the loser. I'll give it a 9.5/10.