I have a 75 gallon tank which I dose Seachem's line of ferts. These include flourish, potassium, iron, trace and I rarely dose phosphorus & nitrogen because my tap water has decent level of nitrates and phosphates (depends on weekly tests). I do have have a minor algae issue but overall the plants are doing fine.
I agree that dry ferts are a lot cheaper and last you longer but Seachem's ferts will do the job as well. Just ask me, I have a high lighted tank (4 X 54 watts T5HO 8 hrs a day, all 4 lights overlap eachother for 4 hrs only) and pressurized CO2. And given the fact that MY water source has enough nitrates and phosphates, Seachem's ferts are the better option for ME, I know in many other cases dry ferts are the best option.
Just to give you an idea, two days after the water change my nitrates levels are between 20-30 ppm and phosphate level stands at 1 ppm and yes my test kits are calibrated.