If you buy a GPU with the mindset that it will quickly become a last-gen product, then you shouldn't buy one at all. Obviously the balance of the market shifts with every release of new GPUs, and you would want to know what's around the corner so that you can make the best possible purchasing decision. Don't continuously wait for next-gen, but don't make impulse purchases either. There is no correct answer to whether you should buy a GPU right now, but gather as much information about the market as you can, and decide for yourself.
I can tell you that the 20nm Maxwell cards will arrive no later than the end of this summer or early fall, just based on market trend and the fact that the architecture is already functional, as seen in the GTX 750 cards. I wouldn't recommend buying anything other than a 780 if you need it right now, because the 780Ti costs 40% more than the 780, but isn't necessarily 40% quicker, and the Radeon cards are inflated beyond belief. Bottom line is, the high end GPU market doesn't look very appealing at the moment, I would hold onto my money at least until there are some new pieces on the board.