I use PDCs rather extensively. I don't play against the AI, though, and they're not as useful against it. Against another player (or yourself) they're absolutely vital.
I always put a garrison in my PDCs. Always. The smallest ones may only be a single garrison battalion, but it's there anyway so that the enemy can't just drop a company of marines on my outposts uncontested. That said, I usually build them in 3 sizes - 1 Battalion, 1 Brigade, and 1 Division.
The Battalion sized PDCs usually come with room for a single battalion, a missile-defense system of your choice, and an anti-ship system of your choice. Alternatively, you could use boat bays or hangars and have some fighters for a variety of roles. They exist solely to defend lonely outposts, whether that's a mining colony or a deep space tracking outpost, so don't beef them up to the point where they become worth more than the assets they are protecting.
The Brigade sized PDCs I use to defend very valuable colonies that don't quite warrant an entire division. Usually these go on valuable automated mines, or smaller mining colonies with valuable minerals. It's the same concept as the smaller design, only beefed up across the board. The price increase needs to be justified by the value of whatever it's protecting, so be careful not to make these ones too big, either.
Division sized PDCs are for my most vital, core worlds. Put enough barracks on it for an entire division and then add on your anti-ship, anti-missile, and fighter capabilities as required (or as you can afford.) These are for defending the core worlds, so the only time "overkill" is a concern is if you're operating under a budget (time or materials.)
Usually I pair them up with a system defense task group that can scratch those hard-to-reach places in the solar system.