Oathe: I bought a couple of games off of here just now after reading some forums saying it is legit, and I can tell you it is, and you get your keys within seconds. Thanks for introducing me to a new site to sink some money into! =p Just got Kenshi for $5! Knights of pen and paper for $2
Kinguin is not legit at all. They are key resellers who don't disclose their sources, they are not authorized in any way by the publishers, and they don't work with the publishers at all. In short, they are exactly the kind of site that developers warn about. If you buy a game at that site, the developer gets nothing - not a cent. Not even the publisher gets anything. All the money goes to the store, and to whichever dubious source they received the game from. And that's excactly the reason why they can undercut market prices and still rake in huge profits. Imho, if you buy keys from dubious sources, it's not much better than getting pirated games. You can't even be sure to keep your games (see, for example, the recent example of Rebellion deactivating 7,000 Sniper 3 keys that were obtained and sold by unauthorized key resellers).
Rule of thumb for online game stores: If the prices seem to good to be true. they probably are.