I still haven't gotten Dawnguard. I estimate I won't be finished with the main game for another 4-5 months. I wring these games out like a sponge, really take my time and enjoy them. But yeah, what will happen is there will be a GOTY edition, probably by next summer. I'll finally pick and up play Fallout 3 while I'm waiting for that.
I just started playing Skyrim again after a 8 month hiatus. I'm running about 40 mods and it looks and play amazing. Picked up Dawnguard but I haven't even gone very far with the main story yet so I can't comment on that expansion at all. Forgot how fun the game was.
Thus far I've not installed any mods at all, not even the official Bethesda stuff. I generally don't go looking for that stuff until I'm out of quests.
You're missing out. The graphics mods, civil warfare mod (Imperial vs. Stormcloak), extra wildlife, harder dragons, more spells, more armor, more weapons etc.... all amazing.
The dragons are pussies, I will say that. I can turn a frost/blood dragon to ashes in about 30 seconds.