I've got mixed feelings about the pack, with the caveat that I'm just getting started on recent modpacks in Minecraft.
At the high end, it's a very good setup. While the tech tree really needs some equivalent to Thaumcraft's manual, especially with all the multiblock structures, there's a clear system of progression that provides significant rewards without seriously trivializing other mods. The underlying concept -- that IndustrialCraft2's fab puts a hard limit on use of vanilla components as a limiting tool -- is not entirely bad. There are actually reasons to consider a nuclear power plant, or mess around with Railcraft, instead of just hooking up an Advance Solar Panel or Magma farm and calling it a day. It's gotten me to explore a number of machines and concepts that I'd never have messed around with, otherwise.
A lot of the pieces are very nice. The Rock Cutter on its own gets rid of a huge number of wasted enchant books worth in the mid-game. Nuclear power provides a good bridge between always-on positive-energy loops (or cheese like lava farming). Its generators fill a big gap in EU production from fluids, and there's starting to be enough automatic machinery to have entirely EU-based industrial blocks.
On the other hand, it doesn't work terribly well at the low end. A lot of design decisions are based around reducing access to resources and especially renewable resources, which is to some extent necessary to avoid the Giant Solar Farm issue that plagues base IndustrialCraft. On the other hand, it does this by raising them up the tech tree, rather than increasing their costs to match the value of the machine. The Solar Panel, for example, gets a lot of flack, because people love renewable power, and GregTech puts it fairly far out of hand. But it's not costly to make -- three glass, eight coal, a pile of flint, and two tier 2 circuits aren't exactly bank-breaking, and the device can repair for those direct costs pretty quickly. It's costly to set up, especially if you use cheaper tier 2 circuits, requiring a huge level of infrastructure, which means it's only less available to the people who would only use one or two. Higher tier Solars complicate the issue further, especially as GregTech's very own Geothermal Generator makes near-infinite energy available as soon as the player hits the Nether -- if you don't have a pile of Silicon Cells or a Fab, you simply can't make these things, but if you do have the setup to power and supply that, you can make so many that it trivializes even thorium nuclear plants.
Some of the matter is a psychology issue, rather than a game balance one. GregTech makes the Macerator require diamonds and glowstone and lapis, because doubling metal resources is an incredibly valuable thing. But because you can't unsmelt ore if you burn excess, this means that optimal play involves diving for diamonds and obsidian at the very first opportunity, because every ore you smelt before making a Macerator is a major waste of very valuable resources. In solo or small server play, that actually ends up cutting out a number of lower-tier IndustrialCraft items from ever being meaningful, except for as components to large machines. I'd gotten four stacks of iron ore before getting my first few diamonds, so the iron furnace sat like a bump on a log until it was part of an induction furnace (and for optimal play, probably never should have been placed). And the Macerator exists in the first place because IndustrialCraft provides a big sink for metals that vanilla (or other mods that add ores) never needed. In the long term, this isn't really a balance issue, since late-game resource generation is too much faster than early-game, but it does encourage bad player behavior.
And then there's stuff that's just stupid. The Diamond Grindstone recipe provides two grindstones per one diamond and four diamond dust. Costly, since the Industrial Grinder needs many Grindstones, but it's a very valuable and late-game machine so that makes sense. But the industrial grinder needs /three/ of these items, aka, an odd number or a number /not divisible by three/. Similarly, the cupronickel heating coil makes multiples of three of an item that you need four to produce. Moreover, there aren't any other machines that need these components, and most players won't want to make multiples of these machines unless they blow the first one up. Gregtech's not the only place this happens (RedPower's frustrating use of microblocks is arguably worse, since it's not always obvious when a slab can be uncrafted into an ingot), but it's a frustratingly poor decision.
It also doesn't play terribly well with some Forestry stuff, since they're pretty much the only relevant thing that uses bronze. The degree this is relevant mostly depends on your world's oregen config -- IC2 default is reasonably copper- and tin-heavy, and has enough to spare that the bronze casings aren't too costly, but the Forestry (and thus many modpack) settings put out a lot less, enough so to leave people copper-starved unless and until they build an assembly table. I'd rather he have reduced the Centrifuge yield by default, rather than have that as the config file result, especially since that acts as an additional EU sink.
