This brought a big smile to my face, because my gaming group ran through Storm King's Thunder with two players playing an ettin named Mundogar. (I think they basically used
these rules, except they rolled for ability scores nine times and just assigned three of those to physical abilities.) Mundo was a barbarian who served as the group's tank; Dogar was a warlock who picked up an unfortunate love of the spell
blink. I learned how terrible of a tank my cleric was. It was fun!
It would not have worked very well if the group dynamics had been different. My character, Kazune, was a Lord's Alliance kid, very straight-laced and formal (but not too strict to his comrades, because that wouldn't be fun for anyone and also wouldn't help). Mundogar was a folk hero (their catchphrase was "We big hero") and a member of the Lord's Alliance, wearing their emblem proudly on basically every article of clothing they wore (as an explanation for why an ettin could amble into town without stirring panic and mobs of local adventurers); he was sent to help Kazune on his mission, along with the first of many replacement characters for a problem player whose foolishness is a story for another time. Mundogar listened to Kazune, but could end up in trouble if he wasn't watched (like one time Dogar pushed Mundo off of the dock, which lead to some miscommunication about going to a bathhouse that I never heard the end of).
TL;DR: This is 100% playable, if you have the right players. To quote our DM, "It's going to be incredible, I just don't know if it'll be incredibly
good or incredibly
bad."
I'm not sure how to make it work in 3.5 and be balanced, but a DM who's interested in trying it out could probably start with the ettin rules I linked.