I've played Minecraft with mods I installed myself, Technic 1.2.5, and now the various flavours of FTB. Its...a bit jerky, at times, and I do get random FPS dips, but not to the point where it makes it unplayable unless I start going insane with machinery. Running all this on a laptop that's about a year and a half old, and it wasn't one of the more expensive models, either. But, manages to run FTB and Guild Wars 2 decently enough. I have to tune down the graphics settings a piece, but honestly...that's not a huge issue, to me. I can have the prettiest graphics in the world, but if the gameplay sucks, I'm not botherin with said game.
As for 1.5 borking worlds, its possible. I'm sure that for vanilla, Mojang will at least attempt to let you keep your current worlds, mods on the other hand, are another beast entirely. But, like the graphics thing, starting a fresh map isn't much of a big deal, for me. Yeah, I'll miss my GraviSuit and bacon gun, but I'll get it back. With the apparent graphical overhaul Mojang did with 1.5, maybe I'll see a bit steadier FPS. Well worth the cost of restarting, if ya ask me. That and, hey, maybe FTB 1.5 will bring us some new toys to play with!
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