I was already extremely happy with the Sixaxis controller, but I could not resist getting the DualShock 3. Not only do you get the rumble, but also, it feels much better in your hands, even for those games that do not use this feature. The rumble function is fun, but on top of that, it helps with gameplay, because "feeling" what is happening provides a lot more feedback than just watching how your character or car reacts to the environment.
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The biggest surprise of all was that the rumble works with PS 2 games in the PS 3 system. I am currently playing through "God of War", a game which had the rumble capability in PS 2. I thought that it was unlikely that I was going to get this feature with this new controller in the PS 3, but it worked perfectly.