I've just started playing with dartcraft and these are my impressions:
1. It's very powerful early game, especially compared to tinker's construct. Getting a tier 3 book is reaaaally quick and gives you access to Fortune 4. I care less about flying, since IC2 access to flying would be at about the same time if you focus on that instead. Getting Fortune 3 on a pickaxe usually costs you over a hundred levels at average, this cost like 5. Also, it gives cheap access to obsidian mining before you've been even close to diamonds, via the powered drill (which is really really cheap and about as powerful as the diamond drill + can be upgraded). It takes a _serious_ Tinker's Construct's investment to get even close to what is cheaply available through early-game (or early mid-game at most) dartcraft.
I like that modded minecraft is shaping back a bit towards the manual rather than the automated (see TC3, TiC, Natura etc), and someone mining manually should usually be more effective than an automation at the given level of gameplay. However, I feel this is a bit too fast.
2. The energy system is out of whack. Seriously. The force engine gives _way_ too much power for a _way_ to common resource. If the amount of power/crystal was reduced to 1/5 and the output/t was halved I could see it decently balanced, but right now, it's a far too efficient machine. Slap down three of them with an acumulator at your quarry and it will withdraw 3 times the amount of crystals it'll use, at the very least.
My hope for the future is that it's rebalanced a bit, mainly through four methods:
1. Force crystals should be rarer than they are, at least only half as common.
2. Recipes should be a bit more expensive. Fortune should perhaps only give one level per two fortunes inserted, and have a special material that can be used to give one level per material (similar to how blaze rods are better for heat than coal is)
3a. Force engines generation/t should be halved.
4b. Using force crystals directly should yield about 1/4th of the liquid force they do now, but have some kind of refinement system (preferably a manual system built into the mod itself as well as compability with the magma crucible) that allows them to yield about 1/2 of the liquid force they do now.