There's loads of stuff to sink your teeth into. The level cap expands to thirty, it sprinkles new enemies throughout the map - Super Mutant Master, etc. I didn't have the GOTY edition either, so I ummed and ahhed over it for ages, but I eventually bought it, did exactly that, and loved it. New character, new specialisations, new gender, new morality, new everything; dug around all throughout the map, doing a proper completionist run, crawling into every corner of the wasteland and finding all sorts of exciting new quests and places, then eventually (after much delay!) reaching the original ending, and finding I still had a good few hours' worth of content to delve into. Good stuff.
Ron Perlman still chews you out a bit if you send Fawkes in. Says that "a true hero emerged," but I find that easy to ignore 'cos it's so clearly bollocks, he wasn't risking anything more than I do when I cross the road. If risking my life pointlessly is how I become a hero, I don't want to become a hero.
I would say, however, that if you're only going to buy one bit of DLC for your Fallout games, it might rather be worth getting one of the New Vegas four. So, so good. If New Vegas is the film, the DLC's the TV series. FO3's Broken Steel is very definitely an "add on," but the New Vegas DLC comprises four distinct episodes, with their own unique styles, tones, characters, arcs and a storyline that arches over all four, some lovely unique weaponry, costumes, goodies you can carry over... it's cracking stuff. Each one wildly different, too; explores a lot of territory they couldn't have touched in the main game. Weird and wonderful stories, took me a couple of days each.
And I'd also quickly note that they're often discounted in the XBox store. I got Old World Blues and Dead Money two for the price of one. So it can be worth waiting. (Dead Money, incidentally, pays homage to the gas mask zombies from Doctor Who! Neat detail.)
On second thoughts, though, if you're just buying one, maybe Broken Steel would be best, as the other four work better in tandem with each other.
Look at that! We've drifted. Whoops. Sorry, kids.