Lets conject and compare exclusive GTA4 360 DLC with current Elder Scrolls Oblivion expansions.
If the DLC was:
Like the first expansion i.e. Horse armour, new dagger, new buying areas that only add very little, etc. then this is fail IMO. It added 1-2 hours extra and was only just about worth the asking price.
Like the second expansion Knights of the Nine i.e. opens a whole new side quest line, good loot, bosses, story development, this is win for me. It added 4-5 hours extra, very worth it and it doesnt complete the full version of Oblivion, but it very much extended the experience.
Like the third expansion Shivering Isles i.e. opens a whole new world through a portal, several quest lines, fantastic loot, new environments, many new enemies and bosses, story development, this will be an absolute must-buy. It added 18-20 hours extra. I can safely say that those who didnt play Shivering Isles did not complete the FULL experience of Oblivion.
The fact the Rockstar already said this will be 'episodic' and will add around 10 hours, shows this is somewhere between the 2nd and 3rd, which already makes it quite safe to say this will be significant content.