Hi and good question. I found this thread by googling the same question you asked for the iPhone 6 plus I am planning to buy.
the screen protectors I have been using in recent times are the glass ones from spigen with the rounded edges. I have this on my iPad air and I have installed them on my wife's iPhone 5s and the same for my niece's.
http://www.spigen.com/brands/apple/iphone/iphone-5s/iphone-5-glas-tr-nano-slim.h tml
i came across these when I was looking for something for my iPad Air. I bought this and those stated above "just in case". I found out how effective these can be a few months ago when my wife slammed her car door on to her iPhone 5s that had this on. the screen protector cracked. The phone bent/curved about 7degrees because of the impact. Because the phone screen itself did not break but just had a stress fracture in the edge because of the bend apple replaced the phone free of charge. It very much looks like this glass screen protector took the brunt of the impact and saved the phone and me a few hundred bucks.
The apple genius mentioned two things to me:
1: the iPhones are designed to bend quite a bit as opposed to just snap; and,
2: if you are going to get a screen protector don't bother with anything but a glass one
lookin around for a protector for my upcoming iPhone 6 purchase I too can see two or three issues. So far the glass protectors seem to not cover the entire screen as per other responses above. I believe they also go over the front facing camera for whatever that is worth. That may not be anything new. The other consideration is as per drop tests found on YouTube I see that the glass protector MIGHT not at all prevent a screen crack if the iPhone is dropped in a way the edge hits the floor causing the impact to push the screen into cracking. Hence I better get a case too. In that regard I have been happy with the spigen Neo hybrid cases as they for me are great as they are quite slim and light and offer a medium level of protection that is RIGHT FOR ME.
last two things to add re spigen (I have no vested interests in the company - I actually have a stm case on the iPad), when my wife's screen protector broke I discovered they offer customers a replacement at half price if you send the damaged one in (it's on the website). The other point to add is in looking at cases for my soon to be phone I am still waiting for further reviews on these spigen iPhone 6 plus products as I have read in a few reviews that they may still be fine tuning their products as some customers have commented that they might not be as good as previous releases (eg the buttons not being quite right. One user comment stated that spigen was actually going to leave the phones buttons exposed in their upcoming modification). So personally I'm still looking to see what the go there will be
let me also say that, yes I am a fan of spigen and am happy to spruce them and to look at them for my future uses but I have not used any other branded screen protectors so I can't compare them To any other simlar products
hope that helps. Bottom line, my advice is if you are going to get a screen protector get a glass one!!!!! I certainly hope I too find a good option that also looks good and feels good and works well with these new phones and their curved face
Let me also finally say that in my own decision making I have considered not getting a screen protector as as it has been said on previous posts above the iPhone's glass should be pretty strong for most bumps and scratches, etc however my wife's experience surely highlights the benefit of the extra protection taking the force of a solid bashing Or at least ONE solid strike
cheers
peter t