I like the merging idea of the various mods. The extended challenge and time to rule the world are great stuff to me. Heck, Quantum might be OP but what do you call the GraviSuite?
GregTech also seems to be relatively NEW as a mod insomuch as it appears to be maturing far more rapidly in terms of balance compared to other mods that have had a long time to develop their feel. I thoroughly expect GT to find a groove over time. Much like aspects of Thaumcraft 2 were terribly balanced (research in my example), time and pondering has led us to the stellar methods in Thaumcraft 3 that we have now. I think mods are a lot like barbeque or slowcooker recipes. The more time you let it simmer and cook up, the better it gets.
Based upon personal experience, GT adds some neat new ideas and options. It also forces me to work with new mod parts and additionally, takes some real steps towards blurring where one mod begins and another ends. I like that.
And in the end, we are not forced to use GT with FTB anyway. I forget if it was this thread or not, but it has been mentioned the Ultimate pack will have GT in disabled form, thus letting both sides have their cake. The ease with which we can add or remove mods through the launcher (one time only at that) makes this seem a good alternative. And how easy the GT config is to adjust still further makes the happy sauce.
Above all though, stop and think about the horrible complexity mods add to vanilla. GT has nothing on that jump!