This may be the single, best cell phone accessory I've ever bought. I've only been using it a week or so, but I'm SO happy I took the risk and got it. About me: I hate, hate, hate 3 things about exercising with a phone.
1) carrying ANYTHING in my hands while exercising 2) that nasty feeling you get when you start to sweat, and part or all of your arm band is plastic and in contact with your arm 3) arm bands that slide down your arm during a workout or ones that are so tight you lose sensation in your hands after they've been on for 5 minutes
This case solves all my pet peeves!
Product specifics: I have an iPhone 6Plus, so it's relatively huge. The elastic holds the phone snugly in place without being bulky. I don't have a case on my phone, but do have a pop-it on the back, and my phone fits perfectly well in the band. I expect that there's enough stretch in the bands that bigger phones like I have would fit while in their cases, but your results may vary, so consider that when making your choices...
How does it address my peeves? 1) obviously, it holds my phone, so I don't have to. There's also a key slot (though I haven't used it, it seems like it would be fine, as long as you wear the case so the opening of the slot is pointing up) and a velcro tab to hold your headphones when not in use, or to wrap up the excess cable during workouts, so you don't get tangled in the cord. 2) There's no "sweaty plastic" feeling during workouts. The band has no/very little plastic in contact with your skin, and the quality of the elastic/velcro seems to be a step up from the less expensive brands. I expect it will last well. 3) The arm band is soft against my skin, and snug enough that it doesn't slide down my arm in use, but doesn't strangle my arm, either.
In conclusion, because I don't believe this band will fall apart from manufacturing/material defects, I may just buy a second so when this one wears out I have a backup ready. It's already a critical part of my workout routine, right up there with my shoes. Now, if only I can find a sports bra that is as useful and reasonably priced, my workout world would be complete.