I would say I enjoy GT. I do think some things are overkill. I think the bronze recipe from Forestry is a bit cheap if you are using IC2 ore generation, but accurate if you are using forestry ore generation. I never seem to have enough copper and tin using forestry ore gen. I shouldn't be able to do one mining run and have enough ore for every single forestry machine with just a stack of copper and a half stack of tin. I think there should be some trade-offs for this.
I will say I dislike the block-ingot recipes from GT. having to place the items in a compressor to make a block makes sense, it just doesn't seem right since it is counter to the normal vanilla mechanics and some metals from other mods are not affected by this and it ends up being confusing. (it will also mess with the crafter from factorization). I wouldn't mind if some of the rechargeable equipment takes steel to make (i.e. drills). It always seemed pretty cheap to me to be able to make a completely unbreakable pick/shovel within a few minutes of finding iron. That being said, I think the railcraft way of making steel is pretty well balanced in terms of time investment vs output, so I would disable the GT method and use the RC only if it is available. I like putting the ingots in the rolling machine for the metal alloys. It makes total sense, plus I can load up the machine, and just let it craft one after another.
I like the balance of the advanced solars mod that is in the FTB pack. The basic solar from IC2 is fine, but very limited in output. I like the fact that compact solars mod is not in the FTB pack. It makes me actually craft different things rather than just churn out a bunch of solar cells and link together with the correct transformer. It also gives an additional use for uranium.
I have never ever in my life created a quantum suit (except for the 1.2.5 FTB map). I have made nano armor a few times, but that's as far as I needed to go. There has always been a bunch more for me to set up and do than acquire quantum armor. That being said, I think there needs to be a bit more natural iridium ore available if the matter fabricator is going the be the uu-matter route. What happens when a server with a hundred or so different users all try to make a matter fabricator? Maybe the admin does something with the post/mailbox/trade-o-mat to make this possible, but the normal ore gen while sufficient for SSP, I don't think scales very well for that many users. People would have to go way, way out in order to find the ore, or make hundreds of mystcraft ages. Either way, I don't think it will be good for the server. Maybe an alternative way of getting the iridium. Maybe like getting diamonds from the rock crusher. It's possible, but it will take a while.
I do love the industrial centrifuges. It finally gives a good use for mycelium, extra lava, and a few other things. sure it's slow, but It's a start and forget type of application, so I'm fine with that. You just make multiples.
I can't say that I will get to the point where I can make some of the end-game equipment (like fusion reactors), as minecraft always seems to update when I start getting close to "end game", so the bottom line is that I enjoy what GregTech offers. There are a few inter-mod balances that need to happen, but I think the modpack creators have done a pretty good job so far.
<edit>GT also gives a route to process some of the "junk" items from other mods into items a bit more useful (like creating clay from mycelium or dirt) instead of just sending it to a recycler.</edit>