Broken Steel truly adds to Fallout 3's story, and is easily the best add-on content for it.
No matter how the final mission to Fallout 3 was finished (with 1 exception) the game will be continued after awakening from that "final" event. There may be a few different dialogue sequences, but otherwise it will start out the same. Unfortunately for you and the Brotherhood, the conflict between them and the Enclave is not yet over. The main story of Broken Steel is of course revolved around that, and it is mostly up to you to help bring the rest of the Enclave down.
The missions themselves involve a lot of action compared to most of Fallout 3's previous missions.This really is a good thing though, considering the fact that the Lone Wanderer will be of high level and probably have tons of powerful weapons by now. Broken Steel also takes you through some new locations, which are full of plenty of opportunities to explore and collect some of the new items added into the game. The best addition Broken Steel brings in however, would be the level cap increasing the previous maximum level of 20 to 30. This comes along with some new perks as well. With this add-on alone, reaching level 30 will take some dedication, but with other add-ons included in the game, reaching level 30 won't be a problem.
Broken Steel has just a few flaws. Most of them are just typical Fallout bugs and glitches. There is one area that caused some serious performance issues to the game, but once cleared out of that area, the rest of the mission went rather smoothly. The story for the most part is clear, but the action, and particularly taking on the Enclave is much more engaging. Some of the story just doesn't feel right if playing as an evil character, mainly because the the main characters of the Brotherhood disregard your particular actions, especially from the original ending.
All in all, Broken Steel is great. It takes you through some neat locations, adds a few awesome new items, some new enemies, a number of side quest, plenty of intense action, great moments, and of course a more conclusive ending to Fallout 3's story. While the ending doesn't roll any credits or anything, it still feels like a real ending. There are 2 ways to end the last mission, and they can make quite of an impact to the game. And once it's finished, it's back to free roaming the wasteland, and again thanks to Broken Steel there are some new side tasks that can be done.
This DLC is easily worth the $10 if bought separately. If there was only 1 of Fallout 3's add-on contents you could get, this would easily be the one. While the missions don't really last much longer than the other add-ons, they are overall more fun to play, as this is an authentic Fallout 3 experience. Plus the added 10 levels the player can achieve is really nice. Therefore, without hesitation, there is no reason to not get this fantastic, and true addition to Fallout 3.