I purchased the Spigen Slim Armor case the day I bought my M8. It's a really nice case. Volume and Power buttons work with ease, and it feels like it will really protect the corners if a drop happens. It adds a bit of bulk, but it's the bulk that protects the phone from drops.
The only drawback of the case is that it blocks the IR Blaster. If that's important to you, the Spigen Ultra Fit Capsule is a decent choice. I purchased one of those as well, and while the IR Blaster works, and it appears to provide about the same level of protection, the Slim Armor case looks much more classy.