What's it cost to rewrite history? 800 Microsoft Points, or US$10 USD.
What do you get in Broken Steel? New weapons, enemies (well, old ones with "super" and "albino" tags), character perks, and achievements, for starters. New side quests and about five hours more "central" plot. A level cap leap from 20 to 30 to grapple with all the new content. But most importantly? The ending is no more. Kaput. History. Null and void. Pushing daisies.
Fallout 3's original ending wasn't a total disaster, but it felt like a lame shrug and a wink post thirty- or forty-some hours of otherwise exemplary gameplay. After dozens of crazy plot twists and endless days and evenings stomping around Bethesda's darkly ironic, magnificently tailored Capital Wasteland, you clicked on the... and walked into that... Err, sorry, I still can't bring myself to spoil it, so
You've also possibly heard that the main story's ending is awful, and to be perfectly honest, yeah, it's a problem -- not even a choice so much as a gun to your head and an irrelevant cutscene that pays shallow homage to the hell you've scrabbled through. And when it ends, it really ends. No continues or tying up unfinished business or pushing through to the game's level 20 cap.
There's an easy solution, of course: Don't finish the game. There's so much more to do anyway before you chamber yourself like a bullet and pull the trigger on the epically anticlimactic and frankly bewildering finale. So don't do it. That ending will wait, and your appreciation for all the lovely things this game gets right will be better for it.