Lumen output is the most accurate way of measuring potential. Factors already mentioned such as the type of reflector, cleanliness, color temperature ( measured in Kelvins) can affect the perceived light value. IE: switching from a warm (yellow) light of 2,700K to a "whiter" 5,000K bulb can make it seems brighter. Most of those posting here posts are replacing an old T-12 light that is probably putting out 30-40% of it's original rating with a new, clean light and so less lumen rating seems brighter.
You previous post was absolutely spot on, it really comes down to "lumens per watt" a typical 100 watt incandescent bulb puts out 1,600 lumens (16 lumens per watt) a new Cree 18 watt daylight bulb will put out the same lumens (88/watt) as the old incandescent light at around 75% less power draw.
Recently I had to make a decision as to which lights to use for my new accessory building. After studying the light output and initial cost I belied that fluorescents was the best way to go. I installed 6 lamp T8-32 lights, the bulbs are rated at 3,400 initial / 3,100 mean lumens so the initial output is 106/watt whereas the mean is 96/watt. Only the most efficient and more expensive LEDs on the market can match or beat this. Now, one can argue that at some point down the road the LEDs will maintain more of their initial brightness than the fluorescents but I can't predict that outcome at this point.
