Fine then. Frankly, I don't see "challenge" or "difficulty" in what gregtech does. Finding metals in a cave or in a strip mine is a crapshoot in my opinion, and I do not find either entertaining. There is no guarantee that you'll win back resources that you've lost, and indeed I have spent many real life days trying to find diamonds normally.
That is not related to Gregs. Actually Gregs has a nice return once you set it up. The Centrifuge, the Electrolyzer and the Grinder are able to create a lot of ressources if you have enough EU. And well Gregs also makes the Nuclear-Generator more interesting since it adds ways to use all the energy such machines create.
To contrast, I point to Twilight Forest's Hollow Hills. It provides an actual challenge and guaranteed payout of resources in exchange for the cost of fighting for your life against possible hordes of monsters. As a result, I've had a lot more fun mining out a hollow hill vs mining out under my home, even though it's entirely possible that I spent the same amount of time and stone for picks.
Well Gregs is a techmod, while twilight forest is... let's call it adventure-mod. How should a mod that adds machines and recipes be difficult? You will never die from such a mod. Gregs is more about Ressource-Management and Planning. And it provides some challanges with this.
It's difficult because there is the distinct possibility of defeat and loss. With gregtech, there's not a possibility of defeat against... more complex and expensive crafting recipes. It's just more tedious for me.
Well I don't really see the tedium in much things. GregTech utilizes a lot of things in multiple ways. You need a blastfurnace for Steel anyway... why not use it for the mining-drill? It is not more tedious to put 5 Iron into a Blast-Furnace than putting 5 Iron into a normal furnace. The only difference is the output. The only difference is that the BlastFurnace is not avaible as early as a normal furnace, so the mining-drill is not avaible as early as it would be normally.
It is the same with the solars. The Electrolyzer/Centrifuge produces two solar-cells every 100 Seconds. The only difference again is moving the Solar-panel away from early-game access. But the costs of the items do not change that much. It is just a processing-element that is used, which you will need later anyway.
What Gregs does is denying early access to certain items.
It denies early access to solar panels for some reasons. First spamming solar-panels is not that industrious. You are not very involved in this and placing them provides no real challange. Gregs encourages players to use the wide varity of not only more efficient, but also a lot more interesting Generator/Geothermal-Setups. Exspecially with TE they are a lot stronger than solars anyway so no loss for the player if he accepts the challange to build a system that runs well on its own.
It also denies early access to the Mining-Drill and the Chainsaw. The Chainsaw is better than a diamond-sword+axe and it is much the same with the mining-drill. Putting them a bit back is reasonable, although RC still allows early access to them by either killing Blazes early (which at this point might be some challange, since you most likely run around unarmored) or finding steal in a dungeon.
Gregs also nerfs the bronze-recipe from forestry (although I think not in 1.4.2) giving you only 2 bronze-ingots (the 1.4.5 version of FTB seems to disable the forestry-recipe completly so this does not matter anyway). This makes it not any more effective than the old dust-recipe.
While complaining about all the stuff people often don't notice that Gregs makes a lot of things more easy. With Electrum and Iron-Tubes Crafting advanced circuits is a lot easier. You normally need 3 Steps, which are down to 2 + giving you more output. Also every recipe Gregs touches accept all kind of circuits, making Iron Tubes a good replacement although you need an extra machine. Lithium-Batpacks are a good alternative to normal batpacks also.