Caveat.
The makers of this app gave me a code to try it out.
EtherealAudioLabs - Ethereal Audio Player app.

What is it?
A FLAC playing app to use on Apple devices. ipod touch/iphone/ipad.
It also supports ogg and wav.
Does it work?
Yes.
Although I did have some trouble initially adding files via itunes. The app would not open and crashed several times.
I `fixed` this by just adding one track and opening the app then adding further tracks.
-On their website they suggest dragging and dropping tracks I noticed today, admittedly I was using the itunes add button as I do for flacplayer and Accudio apps.
The sound.
Track. The Greatest by Cat Power. Using Fostex hp_p1 and ATH-ESW9.
It is on a similar par with flacplayer and Accudio app in terms of sound quality.
There are slight differences in terms of treble, mids and presence of bass between each of the above apps. Only noticeable by listening critically.
Fostex hp_p1.
Ethereal - Airy, more treble.
Accudio - more micro detail, mids, closest to flat.
Flacplayer - neutral, sharp, vocals.
Headphone out of iphone 4
Ethereal- pronounced bass, treble, recessed mids.
Accudio-Clear, good at high volume also.
Flacplayer-forward vocals, mids. (If using eq to lower gain to -8dbs as accudio app it becomes more even).
Accudio player wins, Flacplayer second, and Ethereal third.
Functions.
There are minimal functions. There is an effect button that I couldn`t tell If it did anything.
Some functions dont seem to work.
A plus is that I can access my mp3/ALAC tracks within the menu also (Accudio app also has this function).
Comparisons.
Ethereal Audio Player $3.99
Basic layout.
Access to FLAC and itunes library.
No frills.
Micro purchases of different `skins` available. Did not seem to work luckily.
Effect button and fast forward/rewind buttons did not work for me.
No track scrubbing.
Flacplayer $9.99
Plays FLAC.
has a 10 band equalizer and gain adjuster.
Fairly reliable, easy UI.
Accudio free version
Play FLAC for free on you iOS device. (It says no FLAC on free but I was able by just adding favorite individual tracks).
Allows you to save one setting for your reference headphones choice.
Access to FLAC and itunes library(see first point).
Can download one free setting to custom set your headphones.
Easy UI.
Accudio pro for $4.99
Allows you to save as many settings as you want.
Access to FLAC, OOG and itunes library.
Easy UI.
EQ.
Can reference set your headphones via their list.
Custom EQ setting.
Accudio wins with Flacplayer a close and honorable second, Ethereal in third.
Summary.
Flacplayer has been around a while has has had many updates and customer feedback and through that has developed into a great app.
Accudio is a new kid on the block developed by the Goldenears group, they have released a wonderful product straight off the bat. I look forward to future updates.
Ethereal Audio Player is new, does its function at the bare minimum. I would like to see more functions such as scrubbing, free skins, EQ.