Well, there is no point in using antimatter solely for power creation (like coal or uranium might be used), because antimatter doesn't exist in the nature in utilizable quantities and has to be produced first, using some other sort of power source in the process.
Antimatter would be at its best as a compact way of storing energy that needs to be released in great quantity and/or at high rate. So an "antimatter reactor" would probably either be a warhead of some sort, probably looking very much like a photon torpedo warhead (just with the magic, supposedly graviton-based Trek forcefields replaced by their electromagnetic equivalents) - or then the power source of a vehicle.
If the latter, one would expect the annihilation power to be harnessed in a fairly conventional manner: it would be collected by some sort of a fluid that would then expand and do work. Tinkering with some sort of a semiconductor arrangement that translated the radiation directly to electricity would defeat the main forte of antimatter - the ability to release energy quickly - and would simply convert energy from one very poorly storable form (antimatter) to another very poorly storable form (electricity).
The first antimatter reactor would probably expand the fluid (which could be the antimatter and its counterpart matter of the annihilation process itself, or then an additional expansion fluid fed into the hellfires where the two fuel streams merge) in a rocket engine nozzle, working in the safe distances of outer space. Later models might utilize turbines for creating rotary motion, or perhaps MHD taps for creating vast amounts of electricity very quickly.
In either case, one might use peristaltic EM fields to spit matter and antimatter fluid droplets across vacuum to a focal point in the middle of a large cavity. Other sorts of arrangement would call for advanced containment techniques that I can't really visualize (but some forward-thinking engineers no doubt are already designing).
In the end, a "linear collider" like the TNG warp core really wouldn't seem out of place as a m/am reactor. The Trek application for the powerplant would also appear probable, although the ways of channeling out the power would probably be differ.
Timo Saloniemi