The H100i GTX is a type of water cooler generally referred to as a CLC or Closed Loop (Water) Coolers. The work by changing the 2 step air cooling method into a 4 step cooling method.
1. Heat is transferred from CPU to a block, exactly like an air cooler (metal to metal).
2. Unlike an air cooler, heat is then transferred to a liquid coolant (metal to water heat transfer)
3. The liquid coolant is then pumped to a radiator where another liquid / metal heat transfer takes place.
4. Finally the heat is transferred from metal to air via pushing air thru the radiator just like an air cooler does thru a heat sink.
The problems with CLCs are:
1. No CLC has ever thermally outperformed an equivalently priced air cooler.
2. CLCs make a lot more noise than air coolers... as in the h100i is 12 times as loud as the Noctua DH-15. Sound levels are equivalent to granny's 1950s vacuum cleaner.
3. You can not add additional components (like a GFX card water block) to the loop.
4. You can not refresh the anti corrosion inhibitors and algaecides in the loop and with mixed metals, (aluminum rads / copper block) ... this is a bad thing due to galvanic corrosion that takes place with wet contact between dissimilar metals.
OTOH, there are OLCs which cost about the same amount with none of these shortcomings. If you think you want the H100i GTX, look at the equivalently priced Swiftech H220-X or better yet the H240-X for $10 more. The H220-X kicks tail thermally, is waaay quieter (H100i is 6 times as loud), is all copper and with the H240-X you can also add a water block for say a 980 Ti at some point and water cool your GFX card too.
http://www.swiftech.com/
http://www.hitechlegion.com/reviews/cooling/liquid/40870-swiftech-h220-x-open-loop-240mm-cpu-cooler-review?showall=&start=3
In terms of performance, well….we could simply leave it at the fact that the H220X is simply the best performing out-of-the-box cooler you can buy today. Period. It slightly betters its predecessor, the H220, as well as the Glacer 240L that is equipped with far more powerful and louder fans. The NZXT X60/61 comes close in terms of performance, but at the expense of far more noise and far less compatibility. 240mm CLCs [like the H100i GTX] can’t touch the H220X in all out performance, and at tolerable noise levels the H220X flat out embarrasses them.
The result of this redesign makes the H220X the best performing cooler that is install ready right out of the box. There is not a 240mm CLC or air cooler that can beat it, and it does it at 20+ dB quieter than the competing CLCs. To sum it up; the H220X offers better performance, lower noise, better aesthetics, flawless design and build, better components and the option of expandability when compared to a CLC. Putting it gently, choosing any CLC over the H220X would be doing yourself a huge disservice.
I think you will get a good read on this from the review here, more so from the comments thereafter
http://www.tomshardware.com/forum/id-2834038/corsair-h110i-gtx-performance-testing-review.html