I got two Spigen bumpers (one 6 and one 6+) delivered from Amazon (Prime next-day) the day before the iPhone 6 release. I figured I'd get one of each so that I could get a feel for how the phone size might feel in my hand. The next day I got in line at an AT&T store at 7:30am. There were about 14 people ahead of me and the store only got two 6+ models. I bought the 6 and installed the Spigen case. The buttons worked fine for me in terms of doing what they were supposed to do, but they felt mushy and didn't provide any clicky feedback.
I got the free package in the mail with the button stickers to fix the clickiness. At first glance I thought, "how is this going to fix anything?" Sure enough, the buttons are nice and clicky now. I'm a little worried about whether the stickers might eventually lose their stickiness and fall off, though.
The bumper case itself seems high-quality. The one on my 6 is a red color, the 6+ model I got is bright yellow. The case is glossy, whereas I might prefer a bit more of a grippier feel, but it still does make holding the phone, especially one-handed, feel more stable and less likely to slip out of my hand.
Some people complain about the little "tag" on the back advertising Spigen, and there's also an indented wording "Neo Hybrid" on the right side towards the bottom. I'd prefer they weren't there, but they don't particularly bother me.
I'm skeptical that these things offer much, if any, protection from drops, but I imagine they're better than nothing if you were to drop it from a short distance and it landed on the phone's side or corner. Again, I think the main advantage with them is that they make gripping the phone feel a bit more stable and less likely to have the phone slip out of your hand. I'm not sure why anyone would want a bumper made of metal. Less likely to absorb any shock if you dropped it and it leaves the phone still slippery.
I will say that $25 for such a tiny amount of plastic seems like quite a racket.