I have the Spigen Slim Fit and was happy with it until I found the Nillkin frosted.
http://www.nillkin-case.com/index.php?route=product/product&product_id=4469
I now have both and only ever use the Nillkin. They're very similar but the Nillkin is very slightly thinner whilst offering great protection and also has slightly less plastic covering the phone, which I like. It also has better grip as the back is textured (nurled) and Spigen is mat-ish finish. The coating on the Spigen has also started to chip/wear after only a short time, which is disappointing. The Nillkin doesn't have any coating as it's through-coloured and the texturing is moulded in so no wear issues at all. The final decider was that the Nillkin obscures less of the screen (by about 1mm each side) than the Spigen and the lip at the front is also lower, making swiping in from either side easier. The Nillkin also works perfectly with the Ultimate Shield Tempered Glass protector as there's around 1/2mm clearance between the protector and edge of case.
One last thing is that the plastic it's made from isn't totally rigid but it is hard-ish. It does have some elasticity, which can only be a good thing. You can twist the case very slightly with gentle force and this flexibility must surely give greater bump protection than the rigid plastic cases which just shatter in impact. It also allows wireless charging as well without any issue.
The Nillkin is the best case I've ever had and perfect balance of protection, coverage and grip and for the money its' unbeatable. I'll be buying these from now on for all my phones.
