The headphone sounds a little bright -- the treble's a little hyped and that leads to some sibilance and potentially a harsh edge when listening to tracks that aren't recorded very well, which unfortunately applies to a lot of today's music.
I compared this to the Jaybird X2 and I thought the Jaybird sounded a little smoother and more natural, but I got a better, more comfortable/secure fit with this Monster. The Adistar's earbud is smaller and didn't create any pressure points like the X2 can.
I also put it up against the Beats Powerbeats 2 Wireless. The Beats sounds slightly better as long as you get the right fit, but I prefer the design of this Monster and found it easier to slip in and out of my ears (I always have to fiddle around with the Beats to get the right fit).
Finally, it's worth mentioning that I've also tried Monster's earlier iSport wireless in-ear headphones and this model is superior to both the standard iSport Wireless and iSport SuperSlim.
Sound isn't fantastic but the fit is secure
In-ear sports headphones are very difficult to do. I still haven't met one that combines a great fit with great sound. Monster's Adidas Sport Adistar gets it about 85 percent right. The fit is excellent for an in-ear sports headphone, battery life is decent, and the included Sport Clip wings really keep the buds in your ears.
If you aren't a stickler for sound quality, you'll think the sound is quite good and a clear step up from sub-$50 in-ear wireless headphones. But anybody who's used to a decent pair of in-ear wired headphones will be more critical of the sound, especially at higher volumes.
That small knock aside, for the price this is a better deal than competing wireless sports models from Beats and Jaybird. It's a shame it doesn't cost $80 like the Monster Clarity HD Wireless does, but there's a price to be paid for wearing the Adidas logo.