It is very misunderstood what people think anti matter is, it has been immortalised in science fiction as some sort of super explosive, very untrue.
What is antimatter first off?
Particles have information contained within them I won't go into detail here you can look it up if you wish, this information is charge, mass, spin or angular momentum, baryon number, lepton number, parity and the list goes on. Now what makes matter different from antimatter. Antimatter is indeed 'flipped' charge (we call it charge conjugation, and is achieved by flipping the quantum wave function in phase space) however this is not all it also has opposite spin and opposite every other quantum number. So it really cannot be more opposite from a matter particle, it is the exact opposite in every single way.
What happens when matter collides with antimatter?
When matter collides with antimatter it is an annihilation reaction, we all know that, but what does it produce? Two photons! Not much, cannot produce anymore or any less due to conservation laws, two photons (however VERY high energy) are produced that travel in the opposite direction to one another. Take a look at some radioactive materials that decay through the beta + route, you dont see them exploding into a million bits even though they are creating antimatter in the form of positrons all the time.
How would we capture the energy?
The energy in a closed system containing these reactions would have no contaminants so absorption of the photons would be low however still present as the reactants would be promoted to higher energies etc. It might be possible to perhaps collect the energy of these photons outside the reactor but Id assume the vessel would just heat up so it would have to be in a similar construction to a tokamak fusion reactor.
How can we get antimatter?
This is the hard bit, our universe is as we know it severely lop sided in matter to antimatter, we don't know why, though we have possible reasons for (K_0 mesons for one if you want to look into that). We know it is more matter then antimatter as we cannot see any annihilation fronts in space which would produce a very powerful emission of gamma rays. Antimatter is at the moment hard to make, is it hard to contain and separate? Certainly not, if we can contain protons and electrons we can certainly contain positrons and anti-protons and its not hard to separate from each other before reacting. To conclude its just too expensive to make and there is not enough of it to be viable CURRENTLY, we may see later another possible method of production then a possibility to this may occur.