Pros - Very smooth highs and midrange. Nice detail.
Cons - Fit is tricky.Sound is so dependant on fit. Hard to insert. Even the smallest tips may seem too large.
I have both the HAFXC80 and HAFXC51. The FXC51 seems easier to insert due to its shape, while the FXC80 seems to have a bit more detail and be a bit more dynamic. I bought the FXC51 first, and didn't realize when I bought it that the FXC80 was only $13 more. I bought the FXC51 based on recommendations here.
These IEMs use a micro driver which penetrates much further into the ear canal than the driver of a typical IEM. This makes the highs much better, as there is less reflection of the sound. It also leads to comfort issues, as the very hard driver enclosure is going deep into the ear. It also seems like different replacement tips won't be so easy to find. with practice, one develops a method for inserting these.
At first I hated the FXC51 due to comfort and fit issues, and without proper insertion there is very little bass. After a while I learned to like it enough(plenty of burn in also helped) that I wanted to buy the FXC80.
These are excellent for classical music, especially for violin and piano. The sound reminds me of open Sennheiser circumaural headphones, even though they have good isolation.
PS. After plenty of additional burn in, the bass is emerging nicely. I should have let these play for a week straight before listening to them.