Most of what's happen to IC2 is the fact that it has become lethargic in innovation... Alot of the new mods right now are pushing onto IC2's doorstep because the one staple component that IC2 had (ore doubling) has been replicated on machines of a different energy system. IC2 also suffers from gated resource syndrome, meaning that if you lack a specific resource (usually rubber early game), you can't take advantage of the mod...
(This is why I'm against all forms of 'resource gate-ing' as a whole. At least Alba doesn't react to that kind of news like Flowerchild did, lol...)
But there are several advantages to having IC2 in your modpack. The first is the quicker energy spinup time. Unlike engines, Generators (of various sorts) push out their power as rapidly as they are allowed to. While BC pipes can circumvent this problem, there is a noticeable delay in engines creating energy than with generators..
The second part, which I can't really stress enough, is the fact that IC2 is much more renewable than BC power. While there are renewable forms of MJ with Redpower (and to an indirect extent, Charcoal + Tree Farming), IC2 still has an impressive renewable system of energy. While Gregtech likes to nerf the crap out of that aspect, I do honestly think that, for the moment, renewable energy with IC2 is superior early game. Once mid game hits, you then divert into Forestry Biomass, or something else that's a bit more robust. But bear in mind, if you need a constant stream of power because you lack alot of disposable resources, then early game renewable energy (not just solar, mind you) can be a godsend. Right now, I'm on a server in a village with 3 windmills at 125 height. It's pretty nice for a low footprint, LV energy source...
Third, IC2 still has infinite tool usage. I'm sure this will change as more and more mods encroach this point, but the ability to charge a Tool back up to maximum usage is incredible. When it was first introduced, it changed the game of Modded Minecraft entirely. Think about it: a renewable iron/diamond pick that you can constantly use. Of course, since the advent of the enchanting system, this ability has come under fire as the IC2 dev team dis-taints for enchantments, but it's nice to have something that only costs energy to use as I need to mine my way out of caves and such.
IC2 has aged well as Minecraft as evolved, but even I can admit it's starting to show its age. However, it's still a good system to base add-on's off of (Gregtech, CompactSolars, CompositeArmor, AdvMachines), just like how Buildcraft has several additions to it that allow for additional modification (course, the only reason they work like they do is due to how 'open source' Buildcraft is in general). I wouldn't write off IC2 just yet. It still has great add-on's and such to make enough of a draw to that mod, which that alone would have been unheard of just a couple of years back. I say, give some time for IC2 to come up with new and clever things. They don't push out content as rapidly as everyone else, but that may have more to do with the number of hands in IC2's development than anything else...
