When I first started playing FTB in the beta pack, I already had my hands full figuring out how to work all the new machines and things. GregTech added an extra layer of confusion on top of that, which got to me after a while, and I disabled it once I switched to the Mindcrack pack. More recently I switched to Ultimate and left it enabled, and now that I'm familiar with how everything else works, I actually like some of the stuff it does. Once you know your way around, the annoyances are easy enough to avoid, and there are a few things it makes quite a bit easier/cheaper (rubies instead of diamonds for energy crystals, sapphires instead of energy crystals for lapotron crystals, lots of great industrial centrifuge recipes, etc.) I'm not all the way to the top of the tech tree yet, but so far it's been good. Playing without it feels like I'm missing out.