In MC1.4 GT was great, lost it's way somewhat in 1.5, completely lost the plot in 1.6 and I became a "#notgonnaplaythatanymore" hater.
However, having tasted GT in 1.7.10 I have to say I'm playing a GT pack and loving it. So I hear you ask, what's changed. For me it's the oregen, what GT has done is taken ores and made veins, we're not talking about clumps of 5 or 6 ores, we're talking about chunks of ores in massive veins spread throughout the world, here take a look at some screenshots of what I mean:
http://postimg.org/image/jbpftnft5/
http://postimg.org/image/a3es6g2q1/
Take into account that in the top screenshot the massive hole I've already dug out was also ore and in some cases you don't even have to go digging for ore veins as they are evident from the surface like this
http://postimg.org/image/nvt722thl/
So GT 1.7.10 promotes exploration, practically nullifies quarrying and brings the mining back to Minecraft in a totally different and unique way. Yes it's all about finding the veins but it's quite fun and once you find a vein you'll have so much of that ore type you can switch to a different height level to go find different ones.
I personally am loving GT in 1.7.10 something I didn't think I'd be saying after the mess that it caused in 1.5/1.6.