Actually some people were against Steam because they rip you off, which is not true, don't buy from them at RRP they will give really good deals On old games or if you buy a bunch of games. Infact heres a current deal.
2k Game Pack, (It says this pack will also include Civ V but then it doesn't list it on the pack, so -_-, but even without Civ V its a good deal, you'd have to want all the games though, its a long list, so I spoilered it. It costs you £69.99, and if that includes Civ5!!! then thats a great bargain.)
Ok, now to my other point, Someone said that most people don't want to use Steam because they have to pay with thier credit card. ??? Most people pay with stuff with Cards at shops, you aren't afraid they will steal your information so why do you think its in jeporady at steam. You can save your information to make future purchases just with a click or indeed you can have it so you need to type it from scratch each time, best to do this if you have other people on your steam account say Children who may decide to buy something without your permission. As for the security of sending your Credit card details over the internet. You don't send your pin code, if anyone gets this information, they can never access your bank, merely do what you did, to buy the game on steam, which is give your card number which isn't more dangerous than using the card at Tesco's. Though while I know, a criminal mastermind with your card number can do anything, but its not likely that your information will get into the hands of someone dastardly.
http://www.valvesoftware.com/privacy.html
This is thier privacy statement, read it if your privacy is important to you. I'm sure all transactions of money are done via secured methods. Or as secure as is possible. Your never safe from faudulant individuals, but Steam should not be more of a concern to you than any other shop.
"When you submit your payment information your data is protected by Secure Socket Layer (SSL) technology certified by a digital certificate."
Steam does use as I thought SSl technology, which is the same technology every payment website uses, it keeps your information as secure as it possibly can.
To go back to the point of why steam is so amazing.
http://store.steampowered.com/app/29180/
follow that link, a game called Osmosis, simple, cheap, glorious, it only takes up 33mb's of space on your hard drive, where would you ever find a game like this if not on steam, I'll tell you, you'd be forced to play a graphically stunted version on flash on your PC, Steam allows small developers to get a game like this out to us, at a very cheap price. £1.74 is all this game cost's, and i'm going to go buy it right now, it looks like it will be my source of entertainment for some hours tonight. Thiers a similar type of flash game like this, I forget its name but thats good fun too

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