CON: Channel mismatch, high-cut above 10 kHz, and poor electroacoustic efficiency(sensitivity) for portable use. Some might find the tonality way too bright, due to the inexaggerated sub-bass & the peak @ 6 kHz.
ON SECOND THOUGHT #4:
The sleeve analysis will be discussed on a separate blog entry. The sleeve analysis won't be carried out, due to the fact that EK5 yields almost no difference in sound with different sleeves. Still here is my two cents for EK5 users: Equip EK5 with a pair of ER6-14 sleeves (for reversed horn-effect) & 1000 Ω acoustic damper, and insert the IEM as deep as possible, preferably past the second bend of the external ear canal. You will end up with much controlled tonality.