Well I can't tell you how big the playerbase on the US version is, but the EU version has about so many players, that you will have about 1 other player in your few range outside town. In front of instances are usually 2-3 people standing around. 5 at max.
I found it okay. If I just wanted to do my quests, there was nobody to disturb me. Enough mobs for everyone. And I can could actually get kills on special mobs that only spawn every 2-4 hours. On the other hand it was still possible to find a party for an instance or group-quest boss by waiting at the entrance or using zone chat.
So I guess you could say it's empty but not empty to an extent where it's not possible to find a party anymore. For me it was just about right.
