i've given it a six. it's really short, and severely lacks side content. i heard 20 hours for the main quest and i did it in around 12, while also doing half of the ONLY side quest in dawnguard, as well as a few other small things. i think they dropped the ball on a few things, one being side content. how do you add at best 15 hours of story, and ONE side quest. one... sure it's got some length to it, but they make you run through vanilla dungeons, that's a mistake.
another thing i was extremely disappointed in was what many assumed to be somewhat of a main attraction, the soul cairn. you spend around half an hour to an hour there. what's the point then? i was excited to enter it for the first time, expecting a few story quests there, few side quests, and then maybe a reason to go back. none of that. you get essentially one quest which is basically "kill these three dudes then kill a dragon" you also get the chance to find a horse, but that's just an artificial way to make you stick around longer, since it forces you to wander around until you find the damn thing. all in all, bethesda really missed a good opportunity here by making the soul cairn a place you spend a short amount of time in once, and then never return.