Something it can be hard to "get" at first is that set bonuses are in a way very subtle.
Example, I have an attack that has six slots. I elect to use 2x Acc IO, 2x End Red IO, 2x Dmg IO.
Pretty easy to figure out what the numbers are going be here. Very straightforward.
Now six-slot it with Crushing Impact. I have enhanced in the following ways:
Acc/Dmg
Dmg/End Red
Dmg/Rech
Acc/Dmg/Rech
Acc/Dmg/End Red
Dmg/End/Rech
Notice that regardless of the numbers involved (which are going to end up better than generic IOs), I've enhanced 4 attributes (notice I'm enhancing my recharge time) compared to 3 with Generic IOs.
Here's the subtle part. I now hit more often (acc), with more attempts (rech), for more damage (dmg), and can do so longer (end red). You don't have to sacrifice any one area (such as giving up recharge in the above example) - you get your cake and can eat it to.
Think about this - Let's say I have 3 slots and put 1 end red and 2 dmg - that's great....*if* you hit. If you miss, those 2 dmg enhancements just went to waste.
On the other hand, throw in Acc/Dmg, Dmg/End, Acc/Dmg/End and you're better off, even if the numbers on one specific IO are worse than what you get with a generic IO - the sum is greater than the parts.
And that's not counting set bonuses. +7% accuracy, for example, doesn't sound like a lot. But let's say you have 4 melee attacks with 4 slots each. Put a set of CI in each and now you have +28% accuracy to *all* powers essentially for free.
There are some limitations on set bonuses, which are outside the scope of this post, but just remember this:
Sets are good, mmmkaaaayy?