well, this is an old posting, but since no one answered...with a 4 foot fluorescent tube you would divide the lumens by 2 if you were using the shoplights that hold 2 tubes to get a pretty accurate lumen per sq ft ratio.
In my opinion cfl's are good for anyone who wants to learn to grow, stealth setups, and anywhere you just dont have room for larger lights...
This plant was grown under cfl(6500k) for veg and (2700k) for flower in a 24inch X 15inch grow area and about 28 inches of vertical room.
Of course it would have been better flowering under an HPS and vegging under a MH. But there is no sense saying they are useless.(For some individuals expectations they may be considered useless). None the less you can grow quality plants under Cfls(and if you can't, then it is probably the grower)
Honestly my plant in the picture turned out much better than the plants my friend grew under HID lighting(MH/HPS) of course that can always be due to strain and the care you give the plant.
Based on that scenario I could say Cfls are superior to HIDs...but I am not that ignorant, so I wanted to ask do you(those of you who think Cfls are useless) even understand what you are talking about?
To anyone who would like to argue that there are better lighting systems, it's not that I disagree. I just don't see a need to say one light system is useless because you prefer another.