For political intrigue, I prefer the Dominion. I think they are the perfect enemy for Deep Space Nine. You start out talking about how a society recovers from oppression and then put the Federation in that situation--see how Earth and the Feds deal with this crisis and if their rhetoric at the beginning of the show matches the actions in that same moral dilemna. Having an enemy that is essentially a dark Bajor--oppressed people want to control as much of the galaxy as possible because "what you can control, can't hurt you" is just the beginning of the onion. They have no respect for humanoid life, mono-forms or solids, and to show that, they create solids to be soldiers and politicians to do their bidding while they are recluses.
But the Borg represent our society as well. It represents our technology and globalization run amok. How long is it before we have interfaces in our brains, carry our computers everywhere we go? Isn't that the next step after the Google glasses? So it could be on the horizon. I see the internet in it as well. We don't group-think or move it with our minds, but we do a lot of research and communication online. I'm doing one right now. And how easy would it be to cause a disruption to that network the way they did in BOBW? How much would we lose in our society where everything is paperless?
The Borg cannot be reasoned with--they are direct and to the point. You will do this or you will die. We are stronger than you. And individualism means nothing. There is no representative to sit down with and negotiate a peace. That is the reason that they are so dangerous--powerful and the collective mind cannot make a decision to respect your border. They are improving the galaxy. If some of this sounds like the rest of the world looking at American intervention, it should. I think it looks at the downside of trying to liberate people to capitalism and technology, especially with the barrel of a gun. It's at least a possible interpretation, not necessarily dead-on.
I think they are overused in Trek, and because of that, it's harder to see them as a menace, especially when we can now talk to the Borg Queen. So a little of the danger is missing now. They can enter into deals with the Federation (or at least Voyager). I think they are the perfect enemy, but that is what makes it difficult to keep them around and not ruin the Star Trek universe (or the Borg) by using them too much.