I've tried two so far.
The first one I picked up is the
Marware Sportsuit Convertible. It bills itself as an all-in-one (it's a sleeve! it's a belt-clip! it's an armband! it's a breath mint! it's a candy! tastes great! less filling!) but while I found the armband function to work very well, I wouldn't consider using it bandless as my primary case - something I had hoped to do before I got it. The belt-clip is just too flimsy, and the case itself isn't very stylish. Plus, the clear plastic that protects the screen (it's an integral part of the case) causes some bubbling/rainbow effect on the screen of the iphone because there are some areas where the clear vinyl touches the screen and others where it doesn't. I do feel like the phone is very protected within the case while I'm running, and the armband is very comfortable.
The second one I picked up is the
Belkin Sports Armband with FastFit. Unlike the Marware, it has reflective piping around the edges, which is nice. It's also made of a more stretchy material than the Marware. That, for me, is key and is the reason I picked it up: I keep my phone in a case (the
Switcheasy Capsule Rebel) and I don't want to have to take my phone out of its case each time I want to go running, to be able to insert it into the armband. With the Marware, there's no way I'd get a cased phone into the Marware. With the Belkin, it's a tight fit but just-possible. (The phone would be snug caseless; it's not like it would be loose within the Belkin, otherwise. It's just that the Belkin is stretchy enough to accommodate the case (barely)). The armband for the Belkin isn't as nice as the armband of the Marware. The Belkin didn't come with a beltclip, but did come with an attachable earphone cord-wrapper. The Marware came with a screen protector; the Belkin didn't. On both of these, the armband comes between your arm and the phone, so I'm not particularly concerned with sweat affecting the phone.
I think that, overall, the Marware is the nicer case. But because it can't fit my phone without removing it from its case, I'm going to be sticking with the Belkin.
Okay: breaking it down -
Sportsuit
Can't fit a cased phone
Flimsy beltclip
Better-feeling armband
Has a detachable key/change pouch
Can be used independently of the armband as a regular case
Clear vinyl screen causes bubbling/rainbow
Comfortable armband
Material is less stretchy
Comes with a screen protector
Overall the nicer case
Belkin
Can (barely) fit a cased phone
Has an integrated slot for a key
Has a detachable headphone cord-keeper
Reflective piping
Clear vinyl screen causes bubbling/rainbow
Serviceable armband
Material is more stretchy (kind of like the nike+ armband)
No included screen protector