I just started playing this a couple weeks ago and wanted to give my opinion on what I've seen so far. I found this by watching the first couple episodes of Direwolf20's LP of it and thought it would be fun based on what I saw. I decided to try playing on hard difficulty, since I've always played MC on hard since the day I started playing back in 1.3.2. I've only made it to the rotunda so far after restarting several times due to having to update the pack on the crappy curse client (its a pile of crap compared to the FTB launcher). Here's my opinion so far:
Map Design: 10/10
The base and all the areas I've seen so far have looked great. I've found all the trophies that are just lying in the open, and a bunch of the hidden secrets. Some of them were in really obscure places, which is a plus since you actually need to look for them. Not much I can say on the negative side here.
Difficulty/Balance: 0/10 (as far as hard goes. I haven't tried easy or normal yet)
This is where things get bad very quickly. The mobs are completely overpowered to the point of encouraging the occasional use of peaceful mode just to get to a grave. I just had to do this for the first time today while attempting to get my stuff from a grave on top of the middle ring of the rotunda. I died trying to light the place up, and it took me and hour and 45 mins of fighting with everything in the book from swords to lava to florbs, punji sticks, ineffable glass, among other things and nearly 200 deaths before I got to the point of feeling justified in using peaceful mode, as progress was virtually impossible with hordes of multiplying uber-zombies with 90+hp each murdering me the instant I got near my ineffable glass barrier. Some were even completely invisible and/or unhittable. Talk about unfair. Tthey can glitch through walls and hit you when there's 50 zombies crammed in such a small space, or they simply spawn on your head due to the broken zombie horde vanilla mechanic. I retrieved my stuff and turned difficulty back to hard and attempted to progress with lighting up the rest of the area with no luck. I gave up after 2 more hours and torched the last small sections in peaceful, as they were hyperspawning ubermob 9000 zombies and skelies with fire or explosive arrows. Seriously, how is anybody supposed to get past that when you die just by getting anywhere near them?
Miners' Delight was so bad it took me 15 hours of gameplay to get all the valuable materials I needed from the bottom of the area. I didn't even visit the middle layer of the biosphere because there were way too many mobs to be able to progress at all. The inclusion chamber was fine though. Even the compression chamber only got me a couple times before I finally decided to put all that free ineffable glass to good use.
It seems like there is no sense of game balance in this pack at all once you hit Miners' Delight though. All the mobs are seriously OP compared to the gear you have available at those points. I don't even want to know how bad it gets later on.
Quests: 6/10
After getting to Miners' Delight, most of the side quests that have opened up seem completely pointless. Why would I need to make a bunch of stuff that is only useful in multiplayer when I'm playing singleplayer? The quests now all seem like they are either "get this ore" or "make this mostly-useless item or alloy". If the whole thing was like the first section leading up to the biosphere it would be a little bit better. I imagine bake to win is the typical AFK for weeks on end grind that seems so popular in games these days for some reason.
I'm going to keep trying to progress on hard until the game gets to be 100% impossible, though I feel entirely justified in getting graves using peaceful when the OP mobs make it completely impossible otherwise. I'll probably try dropping to normal once things get impossoble on hard. I would definitely look into rebalancing things on hard though.
I'm hoping ME^3 will be better, as I plan on giving that one a shot to see how this all started out.