I was one of the few that purchased the Series 5. After going onto Apple's website and comparing both models the only thing I am missing is the blood oxygen sensor and I don't think that is worth shelling out $430 plus tax.
I feel the same way. The new blue color looks nice, but my S5 works great for me, and it's not worth it to sell my S5 to get the S6 purely for a new color.
Although my wife is probably going to get the S6 since she will be coming from the S4.