Death Note: Relight is another telling of that DN story. We all know how it goes. Boy meets Shinigami, boy uses a death note to kill numerous people and become a god...yeah. We get it. We have watched the anime, read the books, and seen the first two movies. You don't need another DVD, guys, seriously.
DN: Relight is basically Ryuk (the Shinigami) telling about everything Light did. You get to watch the entire series all over again. It could have been fun.
Pros: Like I said, you can watch the entire DN series all over again. Occasionally the way a scene was viewed is changed. Some people might enjoy that. Also, a few of the random things various characters did was changed. You can see what Ryuk has been up to since the end of Death Note, in the Shinigami Realm.
Cons: They changed a bunch of things, skipped some parts, and cut the ending. Yes, you read that right. They snipped part of the ending, and actually made it seem like the opposite of what actually happened. Apparently, there's a second DVD coming out October 27, 2009 that will tell what happened next. They didn't really show Light's change, how he had good intentions and slowly became corrupted. He actually comes off as insane before he actually was.
My opinion? Don't watch if you haven't seen the original DN. Go watch the full version. If you have seen it all before...maybe you'll find it interesting. Who knows?