Incredibly smooth, yep. Few downtimes for patches etc but just enough time to make me go get something to eat etc. Only issues I have found at all annoying are:

grouping is buggy and horrible xp
some quests (at least in the stygian homeland) have long drawn out chains where you only get xp on the very last step, and its the same amount you'd get from doing a non horribly convoluted quest.
Occasionally monsters that aren't part of a group will aggro from insane distances away for no reason.

Considering it's not even really out yet its a very well done release though! I think they are trying to make up for the reputation they got with there anarchy online launch (two hours stuck in a wall anyone?). This was really smooth and the gameplay is fantastic
If you can swing it upgrade your video card so you can run it at full settings, its really worth it