I read a thread earlier on this board about people with skimmerless tanks who have had a lot of success, but I never had success without a skimmer. I starterd with a Sea Clone and always seemed to have trouble keeping my water "right" After I got a Euro-Reef, I noticed a huge difference. I do not test my water parameters as much as I should. I know I'm gonna take flack for this, but I just look at my corals and clams and such and see how they look. If the tank seems to be doing well, then....
But back to the topic, IMO I think a skimmer makes a big difference.