Good, entertaining watch despite its many flaws.
Finally got around to watching this series after hearing so much about it and my overall opinion is that, for a show that is poorly written in so many ways, it is surprisingly compelling and hard to stop watching.
First, the bad. The dialogue is terrible. There's no way around that. The writers cram so much exposition into everything that's said that it is a rarity that anything any character says sounds natural. Light's inner monologues are guilty of this as well and, especially in the early episodes, he can't seem to hold a conversation without going into deep thought in between every line of dialogue. This is almost forgiveable in the first episode or two as the writers have to explain the concept and the character in a very short space of time but it got to the point where it became ridiculous; someone would ask him something completely trivial and he would start considering whether or not the way he answered would reveal the fact that he is Kira for example his Mother would ask if he wanted something to eat and he'd stand there for about thirty seconds thinking something like "If I say no. She'll think something's wrong and only naturally assume that I'm in my room killing people by writing their names in a notebook." Obviously, that's an exaggeration but it is essentially what those interactions boil down to. I would love to see an edited version where we don't hear Light's thoughts and so after a question is asked he would stand there in silence for thirty seconds before answering usually with a one word answer.
A lot of the show centres around the intelligence of Light, L, Near and Mello but they only appear intelligent because they come to completely insane conclusions that just so happen to be right. Some of their deductions are completely ludicrous and would be impossible to work out but the writers just give them knowledge of things they can't possibly know and pass it off as them being super-intelligent. Well to me it just seems like super-lazy writing.
However, the show manages to remain entertaining through its premise, its interesting showcasing of morals, and most of all the dynamic between Light and L. Which is why the writers should have found some way to keep L in the show. There are legitimate reasons why L was killed off and it made sense in a lot of ways but after that the show couldn't replace the void his absence left and instead replaced him with close to carbon copies of himself with Near and Mellow and it just seemed incredibly forced.