GregTech is, for me, one of those things that, when configured properly, can introduce new challenges and hurdles to overcome while not explicitly making everything as difficult as humanly possible. The chemistry work alone that you can perform in GregTech is, quite honestly, incredible. I haven't played with MineChem, so I dunno how it stacks up against GT's chemistry bits, but I like how you can, with the proper machines, turn even relatively useless garbage into something useful. Have an excess of clay? Centrifuge it, and turn it into lithium, which you can use to power liquid-fueled boilers.
My only real gripe with the mod, in and of itself, is that some of its machines, specifically the inventory management machines, have a far steeper learning curve than they ought to have, and part of that is due to the GT wiki being somewhat lacking in the finer details and workings of those particular machines. This, as one might imagine, leads to either a lot of researching, or a lot of guesswork. Though, if you ask me, I kinda enjoy having to experiment like that; using the empirical method as opposed to having all the answers right there at the tips of my fingers.
GregTech, as is with every mod for every game that ever has been and ever will be made, isn't for everyone. Some will enjoy it, some will tweak it to make it better for them, and some will outright loathe it. That's one of the main lures of the FTB Launcher: you have options. You can find a pack you like, or modify one you've already played to suit your tastes. If you ask a thousand people what they think GregTech's role is in a modpack, you'd likely end up getting a thousand different answers, and I wouldn't have it any other way...minus the pointless flame wars and such that occasionally pop up.
As for a less important, half-joking gripe with GregTech: the seemingly random mashing together of words for machine names, like "Fusionreactor." I can see it just being a space thing, not wanting names to be too long, and it is still technically readable, I'm just a bit OCD when it comes to grammar and such, lol.