My current phone (EVO 4G LTE) has Beats native to the OS. I can say for certain that it makes an "audible difference." Whether that difference "improves" the sound or not is a matter of opinion. The vast majority of my music is ripped straight from pristine CDs using EAC as FLAC for archiving on my hard drive and then converted to 320k mp3's for use on my phone. I use Sony MDR EX300 in-ear headphones - not top-of-the-line by any means, but very good for the price.
So, on good quality music files with good quality headphones, Beats absolutely does make a difference. As implemented on the EVO, there are 3 Beats settings to choose from: Solo, urBeats, and other. Other does not make much of a difference, but the other two do, and seem to be tuned for either over the ear headphones (Solo), or earbuds (urBeats).
Overall, the "difference" is that it boosts bass and treble, as if it's trying to restore what's lost in compression. On some tracks it sounds good (predictably hip-hop, techno, some industrial, some rock, metal is a mixed bag); on others, it sounds like crap - introducing audible distortion and muddying the overall sound. Those of us of a "certain age" will recognize this as being almost exactly like the old Loudness button from the 80s and 90s.
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