(Core Set (beginner) perspective)
Deprived of all the fancy options players get in later cycles, Magnum Opus shines through in the Core Set like a gigantic glowing middle-finger to Corps who not only think they can be ahead on money, but also that they can rely on the sheer presence of ICE to protect then. As most games start out, you will put down a few assets, maybe play a cheeky hedge fund and advance some agendas, all behind some basic but affordable ICE to tax the runner and keep them in check as you contend for an advantage.
Then along comes Magnum Opus and says, "Those are nice ways of making money and all, but here's the thing...I am money."
And it's not lying. It is money. Pure, infinite money. Therefore...
It will remove most of the lasting tax from your early-game ICE.
It will, with worrying speed, put even the most exorbitant toys within reach.
It will allow the runner, even at low link, to comfortably contend with most traces.
And it will, barring any mistakes, never run out, thus putting the Corp on a slow but steady countdown to being royally f*cked.
Its one explicit weakness is that at 2MU it will need set up to bring the runner's full rig into play. That will, with luck, take some time.
Another implicit weakness is that Magnum Opus is precious, and therefore runners tend to be more cautious about face-checking in the early game, for fear of losing it. This indirectly gives some added potency to your Ichis, your Rototurrets, your Archers etc. Even when they don't reaaaally need it anymore, sniping Magnum Opus from their board can be very demoralising. What's more, since runners often chase after it with such gusto, they (may) have binned their fallbacks while hunting for it in a hurry. For single Core Set players there are only two available, after all.
You must therefore seize any opportunity in the time between them getting Magnum Opus and their full breaker suite (or vice versa), to get ahead. Be merciless when evaluating your ICE. Remove what no longer works and use the install savings to get down bigger, late-game ICE, something to at least make the runner spend time clicking back up to break. Any time you give the runner to relax is time they can spend casually clicking for credits. Stall for too long trying to find a vital card and you'll have allowed them to amass a pool of cash, with which they can walk into practically any server you have, big ICE or no. Especially if you're playing a deck that is thin on traps, you will have to just keep an eye for openings in the runner's momentum.
In this way, despite the frustration you might feel at seeing this, it is an excellent card to play against, because you will become a better player trying to beat it. You will learn to value the time before it appears, and how to be efficient once it has. You will learn to make snap judgements, cuts, bluffs and gambles as the board adjusts to its presence. And you will learn to remain calm under pressure, because in the Core Set at least, it rarely gets more pressuring than this. I suppose in this way, it's actually been one of the most valuable cards for me to start learning with, for two completely different reasons.
So, thank you Magnum Opus. Thank you on the one hand for giving me all the money in the world =D And thank you also for teaching me how to play the Corporations better, you horrible, triangular bastard.