(Core Set perspective!)
Short for 'You Old Git', a perfectly acceptable response when someone looks at you, grins evily and makes a slow unlocking motion in your direction. Feel that? That's your clever puzzles and sharply-dressed headshot bots doing nothing, absolutely nothing.
There are a lot of moments in Netrunner that will make you appreciate the difference 1 can make, and not just in the useful trickles of money you get from doing innovative things like running or putting a card on the table. No no, I'm talking about the difference between 4 and 5 when you have a Sure Gamble in hand. I'm talking about having 9 when someone Forged Activation Order(s) your Hadrian's Wall. And in this case, I'm talking talking about the difference between 1 and 0 when someone wades through your carefully-placed stack of puzzling defenses.
The thing is, even with the most m4d pr0 breakers installed, the Runner will in most cases still have to pay something, even if it's just a sympathy credit or a power token. Sure, a Corroder shinkles through fuzzy walls for a measely 2, but damn it, that's 2! That's 2 they don't have any more, or 2 they had to flip over on a Cyberfeeder (I don't care! Let me enjoy this). If there's another wall behind it, that's like, 4! (100% more than 2!) You might think, 'I don't feel so bad now. Sure they stole my second Priority Requisition and I've got three agendas in hand, but they at least paid to get it.' And you'll hide a little sniffle behind that brave face of yours. You really don't appreciate this small consolation until you have a YOG go through your stacked Viktor 1.0s to yoink something out of your hand, for free. It's like a ghost just stole your lunch money. You're not even hurt, you just sort of sit there and go, '...oh.'
With a Parasite or Infohoover powered up as well it can make your life very miserable (or great, depending on what side you're on). It's absolutely worth the 5 cost and after a few games you'll instinctively know when it's coming, and after a while you won't even fear it, you'll just sigh at the inevitable and put down a sentry instead.
Now a of couple quick tips for playing against YOG.0:
As said before, as a general rule don't stack code gates. Spread them around. Show them the world. Let them live a little. Losing a few defenses on several servers will in most cases be preferable to losing everything on one.
Keep a Corporate Troubleshooter handy to surprise boost one of the various program killers you hopefully have installed. Raise an Ichi 1.0 or a Rototurret above their ability to break and you'll get a shot at it, and it's so satisfying when you do. Then you'll be sad when you realise you only have one of these guys in your Core Set.
Remember you can purge virus all virus counters for ,, (not one at a time, like I thought originally), which resets the threat on a lot of things YOG needs to overcome ICE it can't break on its own. It won't remove the problem, but it will give your some breathing room.
Sometimes, especially in a tight game, 5 can be hard to find. Prolong your safety from YOG by keeping the runner pressured. Install, advance, double-advance. Keep them guessing, keep them nervous, and keep offering bait that they'll pay their precious life monies to check. As long as they're doing that, that magic 5 will be further and further away.
If they've come to rely on YOG (which is easy to do without realising), a sneaky Aggressive Secretary in the late game can wallop them right in the prospects, giving new life to those code gates they'd written off as worthless, and opening up a scoring window you need to secure the game.
To sum up, in the Core Set you play your code gates knowing this will come out eventually. At 1-influence it's an obvious choice to include, especially in Criminal who don't actually have a Decoder of their own. It's a great thing to have and great pain to play against, but sooner or later you learn to deal with it. Fact is you won't have a choice in the matter.