Insurgent starts off immediately where Divergent ended, so if for some reason you read Divergent first, you're kind of hosed. Even having read Divergent fairly recently, I couldn't always remember who everyone was in the first half of the book, and then in the second half I didn't always follow the action so easily. Sometimes authors get so caught up in their characters and stories that they forget that you don't know and love their children as much as you do. Don't get me wrong; Insurgent is a great read, but it does have that middle-of-a-trilogy thing going on, where even at 525 pages, it's really just a vehicle to get you from the beginning to the end.
The end is an abrupt cliffhanger and will be upsetting if you're a book junkie. The third book is as yet untitled and won't come out until next fall.