Bethesda knows what they're doing when it comes to downloadable content. With the exception of the short periods of glitchiness that plagued both Operation: Anchorage and The Pitt (which Bethesda was smart to fix post-haste) Fallout 3 has had some pretty fantastic additions onto the story of the wasteland wanderer. The only problem is that even with the previous two DLC packs – the story still had a concrete ending, no matter how you played.
Some things change, and some things don't. The Broken Steel DLC released for Fallout 3 changes the ending – by continuing the storyline. What doesn't change? The quality of Bethesda's DLC – Broken Steel is as good (if not even better) than the previous showings.