I have a MOTU 828mk3 Firewire. It has been extremely solid for me and sounds definitely good enough.
The CueMix is really cool because it actually maintains its state after power states and can be run without a computer. The interface is sorta clunky but not awful.
A couple things though....
1. The DSP can only be active on inputs or outputs. So you either have EQ/Compress on the way in and print it, or on the way out if just for monitoring.
ie. Tracking vocals... I want a dry feed but a wet mix for headphones. I have to set up a mix of just vocals... send that to an output that I can EQ/Compress/Reverb up.. whatever and then mix that separately with the mix coming from the DAW. My Behringer heaphone amp lets me do this with the aux... no biggie.
ie. Full band... you will need an external mixer to re-mix the headphone feeds with a bunch of custom mixes for each instrument if you want it wet. Or track it wet, which works well too.
2. The DSP really only has enough juice for I think one band of EQ and compression on each channel, based on the manual. Channel not being defined to include the ADAT channels and such either.
Personally I have never maxed it out. The MOST I have ever run with FX was 8 channels coming and recording it wet. It was for a jam party and I just left things recording... I don't remember all the details... Jameson punched me in the face hard that night. I even hooked up a Behry BCF2000 to it so there were faders, awesome!
Usually I just use it for overdubs. Full bands or drums, I do everything in my DAW with low buffers.
For an in-ear system... I think it would be successful depending on how many FX you think you will need. Album quality I think would take so much that it may not do it. But better stage sound than every F'n venue I have played in should be trivial... and easier to move around than a traditional mixer.