Durarara has a strange style and pace to it. Near the beginning, the piecemeal story is kind of frustrating and the visual style of graying out extras feels odd. Some of the characters feel very removed from what's actually going on in the city and one can be left wondering why we spend so much time on them. However, if you have the patience to get through the beginning, you will be rewarded with an engaging series that's not quite like anything else.
The cast of characters is pretty charming, and with so many, you're bound to find someone to like and root for. Despite all the characters, I never found myself bemoaning the fact that a certain character didn't have enough of a focus. The amount of time spent on each character is pretty perfect in proportion to how important they are to what's going on in the story, so no one ever really feels left out. Everyone seems to be keeping shady secrets in this city, and part of the fun is trying to figure out what they're trying to hide. As the series continues, each character's individual story slowly integrates more fully into the overarching plot. By the end, everything joins together as a whole. Personally, I was gripped to the end and found that all the little struggles were wrapped up quite well. It's easy to become attached to these characters, as the show does an excellent job at making lots of characters easy to relate to or sympathize with.
The art is nice to look at most of the time. The backgrounds are gorgeous and the character designs are done well. However, as mentioned above, I personally found the grayed-out extras took a bit of time to get used to before the scenes really looked "right" to me. As I continued, though, I found it to actually be a huge benefit and a neat stylistic choice. It's really a matter of personal opinion. I thought the soundtrack was pretty forgettable for the most part, though. It's not terrible or anything, but it didn't really stick out in my mind as being really noticeable except for the openings and endings. Fortunately, the English dub and sub are both awesome so just pick your preference.
Durarara is not for everyone. It's very scattered early on and even as it comes together some may find the large cast and storytelling style is not to their taste. But please give it a try if you think it sounds interesting; you just might love it.