For me, the marker of a great game is that I don't want to do something else whilst playing it. There are very, very few games that have achieved this: Dark Chronicle, Bioshock, Fallout 3, and now The Elder Scrolls: Skyrim can join them.
Skyrim had claws sunk into my head so badly that I had to ban myself from playing it during the week so I didn't play past my bedtime and have to go to work on 3 hours sleep for 4 days running. Instead, the first weekend, I managed 18 hours play in 48 hours. Over the next couple weeks I slowly drip-fed myself during the week by indulging perhaps one night in the week. I really, really had it bad.
I've finished several story questlines: Dark Brotherhood, Mages College, Companions, and finally the main quest. I'm not going to spoil anything for anyone, except that I haven't been disappointed in anything so far, except perhaps the final battle against Alduin, who was, for all intents and purposes, just another dragon. But honestly, I can deal with slightly disappointing final battles; I played through Borderlands after all.
I'm now at a bit of a crossroads. I have a significant amount of other quests, both miscellaneous and regular. I can scroll down my list of regular quests for a good 5 or 10 seconds before hitting misc. I am really rather overwhelmed. I have no idea what to do next. There are still a great deal of places that I haven't found yet, and many, many more that I haven't cleared. It's a veritable rabbit hole of a videogame. Almost every time I talk to someone, I get more stuff to do! It's crazy.
Completion is another tricky proposition. With the radiant quest system, there's infinite amounts of quests in the game. So should I just finish all the regular ones? How do you even find all the regular ones? I think perhaps just emptying my regular quest list will count as complete - otherwise I'll be at this forever. And I think I could probably play it forever too. 5/5 stars.