It's a scary thing to see giants die from massive damage; one well-placed fireball wiped out three with one hit. 0.o
Looking at the numbers afterward, it makes sense. Frost giants are CR 9, which means that a single one should be beaten by a party of four 9th-level characters, using about 25% of their resources (hit points, spells, etc.). Our party is nine 9th-level characters, and we fought the giants pretty much in groups of four. Plus ... wow ... that giant bane sword. Man, it lives up to its name.
We have until now been pretty skittish about fighting giants, in no small part because our earliest encounters with them, they smacked us up one side and down the other. They are heavy hitters, so our most consistently successful strategy has been to ambush them and pound them into mash in as few rounds as possible. The longer the fight goes on, the more likely the giants are to land a couple of good blows -- and there are members of the party who won't survive one blow like that, much less two or three.
After all the giants were sent to frost giant hell, several members of the party leveled up, including Theran. This is the first time I've continually played a character long enough to reach 10th level, so I'm kinda jazzed about that. For you oldschool D&D folks, 10th level in D&D 3.x is roughly like 12th/13th level in AD&D; we're not quite gods-among-mortals yet, but we are definitely powers to be reckoned with.