I've been waiting for the S6 to be announced, and had planned to make it my next phone. But now, I'm very disappointed with the specs: smaller non-replaceable battery, no SD card, not waterproof. I don't care about the glass front and back, etc, or whether the back is made of metal or plastic. I buy my phones for functionality, not to compete in looks with Apple, plus, I keep it in a case anyway, so I never see the back. Having a curved phone looks like a pointless gimmick and I don't need a wrap-around screen. The only Pros I see with the S6 vs the S5 are the faster processor(s), better screen resolution and possibly, a better camera. All of those specs were completely adequate on the S5, so I think the negatives outweigh the positives.
This is an expensive phone, and I'm sure that the need to buy more onboard storage is going to drive up the price. I'd much prefer to use SD cards which get cheaper all the time.
So, the bottom line is that I'll be getting an S5 vs the S6. This has the advantage that root and custom roms are available now, so I won't have to go through the growing pains of waiting for S6 development to catch up. I'm really disappointed that Samsung decided to turn the Galaxy phones into an iPhone wannabe, but I guess that's what the mass public will buy.