Classifying Durarara on which genre it belongs to will put the title to different categories. Should it be considered a slice of life, Thriller and Adventure depends mostly on how you viewed it but if you're going to ask me I'll make a different category for Durarara and call it creatives. Stupid as my sugesstion is, it's the best word that I can describe for the anime. And good thing that a friend of mine suggested me to watch it because I just missed a quarter of my life when I didn't watched the show.
If you're used to the normal style of watching anime where everything is linear and complexities build up in the later part of the show then you probably need to embrace Durarara differently. I'd say that the most creative aspect of the anime is on how they jumbled out the sequences of the whole story and as each episode progress everything made sense at the end of the story.
You might get discourage if you're not used to it, same to what happened to me when I was watching Baccano (same author as Durarara) but Durarara's lively style of modern Tokyo separates the two show away from each other. The momentum of the show is really outstanding and the great narration and casual conversations makes it more enjoyable. The first three episodes would give you a solid grasp of the show as you might get confused at first because Durarara is comprised with crazy, out-of-this-world characters that will mislead you on different thoughts of the plot.
What makes Durarara lovable is the huge array of characters and side characters where each got it's own spotlight in the show. In fact these characters are important in making the story interesting and an important part of the puzzle to understand the whole point of the story. Miss an episode highlighting a single character and you just missed a major part of Tokyo and it's significance to the show. One thing that I noticed is I like the side characters more than the major one, with Mikado as an exception. If I'll pick my favorite, I'd choose Shizuo-chan followed by Mikado of course.
Artwork and Soundtrack is among the best I've seen. The details of the buildings to the alleys, parks, sidewalks are of high quality and it just makes me love Japan more. Mix it up with catchy music which resonates in your emotion and you got yourself and entertaining piece of work.
Durarara is perfect for young adults but is recommendable to anime fans. To those who wants mystery, thriller and any genre to date (even hentai!), it's a solid pick! Durarara definitely deserves an A+