If you want quality, avoid keyboard cases. They're all essentially the same. Just posted this in another thread, but I'd instead recommend getting a slim folding bluetooth keyboard:
(
full review here)
I got mine on amazon, but they have a generic version on ebay for ~$30 last time I checked. Believe it or not, that keyboard was $28 when I first saw it, but it jumped to $32 in my cart because I waited a day. And now it's $38

. Still worth it, but if you can get it cheaper you might as well.
With keyboard cases, the method of securing is either: a couple prongs on the top/bottom (unless you DIY like I did with mine, it's unsecure); a couple elastic tabs; or, a faux leather bezel (which I find aesthetically unappealing). If you get a
separate keyboard altogether, you can use your favorite case and not have to worry about the keyboard when you don't want it--e.g. you don't have to switch out cases when you're reading on your bed.
With keyboard cases, they keys are going to be ultra-mini. Mine--the generic one with prongs--had a keyboard that was 8" by 3"--keys were miniscule and cramped. Fine for me because I already type fast and don't have large hands, but it still reduced my WPM to like ~30 as opposed to ~100 on a decent sized keyboard.
Though, I have that "crappy" $10 one, and I used it for a good few months before switching out. It still works great, it's just that my shoddy OTG cable crapped out and I decided to go bluetooth. I took it everywhere, and I never had issues with the keys popping out or anything like that; I'm sure quality control isn't too great, but chances are you won't have issues. Browse
here until you see the modified case pic with instructions--in case you plan on getting the one I did, you really should modify it.