It's still very very impractical. Customer water loops are for hardcore overclockers but for the average gamer it's just plain dumb. Put it this way, he's buy a GTX 780ti cha ching $700-ish dollars. Now he has to go find a waterblock for it, he's found it cha ching another $150. Now he has to go through the hassle of removing the cooler from the card, this risks the warranty on the card and the physical PCB itself as one slip-up could break it. Now he's gotta buy tubing, coolant, a pump and fittings.He's got all that another $200-300. We're already at $1050 or so. Now he's gotta build the loop and be careful he's done everything otherwise one leak could fry his system, okay great! he's done it. Now comes a week and a half of overclocking and stresstesting for stability. Photonboy is right you can buy 2x 780s by the time your done with this. Now 2-3 years later you want the newest Geforce 800s series. You gotta wait for waterblocks to release and buy the new card and the waterblock, drain your current loop and take our your od GPU and put in the new one.
In short custom water loops are for people looking to overclock to the next level and it's only not worth the time for an avid gamer. Time, patience and lots of money need to be invested in custom water loops