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Homepage:
http://feed-the-beast.com/
(Tekkit, but with permission and quality. It is fresh from the mod makers themselves.)
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Previously played on the FTB default Mod Pack (A) that had IC2, Buildcraft, Mystcraft, TwilightForest, Railcraft, Stevecarts, Extrabiomes and etc. It was missing Redpower 2 & Thaumcraft however so haven't used it all that much. Was so much fun until a pack update corrupted my map I wanted to keep forever. Was pissed.
Now using the Mindcrack Pack on a new map with twice the enhanced gameplay experience. Simply awesome and fun; highly addicting. It has what the default pack had, and more. Currently screwing around with the semi-buggy Traincraft mod within it and the Turtles that help you mine. So damn awesome! Glad Redpower 2 was added because I get to now play with marble, basalt, and sickles again. All those circuits and such. Volcanoes anyone? Those were fun on my previous main map.
Got a nice train supply line up and running with the single train I have there. Not just any train, an actual train that isn't a Minecart. It is a steam train requiring fire and water. Love how I can control it and poke it where I want to go. Sadly, I don't know how to make freight cars, have to use storage carts for the time being. Not all that teched up at the moment because I'm more focused about upping a supply line for resources and building a nice route to the east for my Capital City

The things FTB adds (especially the Mindcrack pack) is overwhelmingly fun. The Twilight Forest itself gives you days of fun. All the new mobs along with dungeons is a nice welcoming edition.
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Alright, rambling over....Too addicted. Who else has it? For those that don't should give FTB a whirl. Just choose a pack and let loose!