[CareyPrice31]

Is there a difference between 256KBPS music and lossless?
 
Will I notice a difference on IEM's?

 

[ThumperSD]

Of course there is a difference. You will/will not notice depending on your equipment and ears.

 

[Forcefedfreak]

Not only do you have to know what you're looking for, and your equipment, but also the quality of the production in the first place can make a difference too.

 

[sgrossklass]

See here.
 
The modeling used in lossy formats does not take non-neutral reproduction into account, so things like mean spikes in the frequency response (old Etys were known for this) or specific kinds of hearing damage may make artifacts audible when they normally wouldn't be. Same goes for further processing, like voice removal.
 
All that being said, in general lossy codecs work pretty well, even at half the data rate mentioned.