In theory it would be a dynamite fuel source; annihilation is a perfect 100% conversion of energy to mass, which means you lose very little of the energy in storage, it's also very quick and very powerful which means you can store alot of energy in a very small container.
And by "alot" he means the energy is more concentrated than a nuclear explosive. And by that, I mean on many orders of magnitude more concentrated. From the wiki article on antimatter:
"The energy per unit mass (9×1016 J/kg) is about 10 orders of magnitude greater than chemical energy (compared to TNT at 4.2×106 J/kg, and formation of water at 1.56×107 J/kg),
about 4 orders of magnitude greater than nuclear energy that can be liberated today using nuclear fission (about 40 MeV per 238U nucleus transmuted to Lead, or 1.5×1013 J/kg), and about
2 orders of magnitude greater than the best possible from fusion (about 6.3×1014 J/kg for the proton-proton chain). The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 petajoules) of energy (by the mass-energy equivalence formula E = mc˛), or the rough equivalent of 47 megatons of TNT. For comparison, Tsar Bomba, the largest nuclear weapon ever detonated, reacted an estimated yield of 50 Megatons, which required the use of hundreds of kilograms of fissile material (Uranium/Plutonium)."
In short... antimatter makes nuclear energy look like a toaster. There is a good reason Star Trek used it as fuel.
To get a little more in depth about the 100% mass to energy conversion:
Technically, half of the energy released is in the form of neutrinos, and so is lost. However, since you use 1 unit of antimatter and 1 unit of normal matter, you get the overall yield of the mass of the antimatter used.
2 (1 anti, 1 matter) times .5 (50% lost to neutrinos) = 100%