I have 2 Titan Black Hydro Copper Signatures in SLI and couldn't be happier.
 
At first I went 2 290Xs water cooled in cross fire.  With Koolance blocks I had zero bowing.  But in the end the 290x in CF was a horrible experience.  I was expecting so much but was only greeted with the infamous black screen of death syndrome that so many 290X users are having. I tried it all for a fix... Drivers, BIOSes, over/underclocking, over/under volting, new PSU...the works.  Nothing helped After a month of this crap the 290Xs went bye-bye.
 
Then I went with 2 GTX 780 Ti in SLI.  Nice and Powerful. Cooler than the 290Xs. And No black screens. But I had horrible coil whine. Just horrible.  RMA'd 2 and the 2 new ones whined worse. :(   I also used Koolance blocks on these and had the slightest bowing.  Very slight. Nothing concerning and almost hard to see.  But the whine was too much so adios GTX 780 Ti cards.
 
Enter Titan Black Hydro Copper Signatures.  These arrived last week. Faster than my 2 water cooled GTX 780i SLI (due to better voltage control and I believe binned cores for the signature series) and dead silent.  I love the peace and quiet now.  I also like the 6GB ram should games push that limit. No, not the smartest buy value wise on these cards, but I can tell you that I have no regret and I am now completely satisfied after months of GPU disappointment.
 
As for the cards themselves I have no bending or bowing at all with the Titan Black Hydros.  I think the back plate helps and the numerous mounting points of the Swiftech block helps as well. I found 780 Ti bowed ever so slightly with an after market Koolance block on them.  But it was very slight.  Titan Black Hydros are totally straight.  
 
I think the Koolance (and other brand blocks) post better VRM temps than the Hydro, but the Hydro is very supportive to the card and doesn't bow.  I've found my VRM temps acceptable so overall I am pleased. 
 
 

 
 
Just like I've been contemplating for a while if I should get the 3rd card and then I did, now I'm puzzled wheatear I should put them under water, I hope by the time X99 boards are out I will have my mind made up, so I hope. I hate to see cards that sag, it's hideous. I have seen 1 video with  2way Sli titan hydro copper, I'm sure there are more videos of other cards with water blocks, but the 1 I've seen it, the top card was hanging about 1/4 of the inch to words the bottom card, that just didn't look appealing. Now! why I mention EVGA's water block? because unlike other blocks that covering 3/4 and some not even that much of the PCB, EVGA's block covers pretty much the whole length of the board, it looks nice, but I've heard that they are extremely heavy and seen that video didn't help me been a bit skeptical. I also had few cards in the past that would sag right from the box, won't mention the brand, but that didn't help ether. I know the temperatures comparing to air cooling are  substantially lover,  now I have 3 Titans sandwiched, even thou temperature are pretty good, still there is no denial that high-end card will show its true potential when is under water.
mdzcpa, when you get a chance and if it's not to much trouble, could you post some pic with your  cards setup.Thanks to both of you guy for your input.

Thanks to all for sharing your insights, and in particular -- to mdzcpa: quite a saga you've had, glad to see you've found the solution that suits you!
 
Regarding the sagging boards -- I presume those were without back plates? I see that EVGA Hydro cards include a back plate, giving the card a "backbone" --  it surely helps to support the block's weight.
 
I've been using three 680s with EK blocks and stock back plates, with zero sagging/bending. In fact, the three cards connected to EK 3-way bridge comprised a solid and sturdy block which aligned perfectly with the PCIe slots when inserted into the motherboard. Side note: the resulting assembly was so heavy that I hesitated to stand the case up with it in fear of breaking the motherboard slots :) took good care screwing the whole thing tightly with 6 mounting bolts, but it held pretty well since.
 
Having recently been playing with a regular Titan with EK block + back plate I am also happy with how straight and solid the result is. I wish the manufacturers would make matching back plate a standard component of a water block they sell -- as GPUs get  much heavier with them. Thought I would include this photo just for kicks -- my three 680s with the blocks weigh over 8 lbs

How are EK blocks weight compare to EVGA's blocks. I like EVGA blocks for the reason that they cover the entire PCB, unlike other brands cover only portion of it.
Any pros-cons for choosing 1 over the other brand, cost is not the issue here so if there are water blocks that others would consider the best but to expensive what would they be,  or its more personal preference, keep in mind i never had water blocks so I'm totally new in this area.
Also,  I do leave my PC on 24/7, would that make a difference going water, will that potentially cause issues?

It will probably help, no doubt, but the thing is if you looking to have your components exposed in nice ascetic way than any supporting brackets will spoil the look, but i guess that depends on some ppl's  different taste, I'm just anal and careful with all my stuff, not just electronics, after 20 years of been married, my wife got used to it
 
 

Check this setup with EK blocks, looks pretty nice and straight
I read some reviews on those particular blocks and people seem to favor them, definitely something to consider

I see, yes that is quite nice block and because the metal  doesn't go all the way to power connectors, yet the plastic part covers rest of the PCB might contribute  why this block doesn't bend like I've seen with EVGA's hydro copper, so i assume that would work with my Titan Blacks, would be nice if by the time I'm ready they would have this with nice Titan Black logos on their backplates.
If you get a chance and you have them installed, don't be shy to post some pictures
Was it difficult to install, do you use close or custom water loop, when time comes I'm going to work on some fancy custom loop, my privies water setup only for CPU was h100i that leaked while i was gone to visit my relatives for 2 days, what a mess i tell you that was
 

mdzcpa

AWK16
 mdzcpa, when you get a chance and if it's not to much trouble, could you post some pic with your  cards setup.

here you go.  nothing fancy.  not for show, but highly effective.
 
 
 
These cards are nice and straight
 

Wish I could get my setup to look that neat. But function over form has left me a mess. Looks nice.

still building my new reg and i have 4 titan black hydro coppers but with ek wb heres a pic or 2