I hate let's play videos. I don't care about some dude with a capture card's opinion. If I want to play a game I will either buy it, borrow it or try out a demo. If none of those are viable at the time I wait for a price drop and pick it up then. Once in a great while I will watch stuff like people finding glitches in games like Pokemon for fun or a walkthrough for a specific achievement I'm having trouble with but if I wanted to watch someone just sit and play I have a 7 year old I can sit and watch. He's more entertaining.
That said, Youtube is a wonderful thing. I can see the value in what some of them can offer. I am just not a fan of let's plays obviously.
i'm not when it comes to watching the entire game, but i don't mind them as a way to get a quick feel for the game itself. you can see how it actually plays instead of watching cherry picked clips in reviews or trailers.
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