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Well they are about an order of magnitude below state-of-the-art batteries in terms of energy density, and about the same distance above batteries in terms of power density. For phones this doesn't seem to make sense unless you want your iPhone to be 3cm thick. Maybe an application with greater power demands?

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Can you explain the difference?

Energy density is the energy stored per unit volume or mass. Power density is the same thing, but per discharged per unit time. It is essentially a volumetric statement of the internal resistance of the battery or capacitor. Supercapacitors can be discharged very quickly compared to batteries of the same size, but the same sized battery contains more energy.

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