I use the Traveler-mk3 for just about everything. I play e-guitar through it so that I can use s/pdif to interface with an MX200 and then listen on the headphones. I run 3.1 channels of TV through it (optical in only decodes 2 channels though - had to use line-ins) too Yamaha HS monitors. I run computer audio (iTunes, YouTube, ...) through it.
But there are problems (at least with my unit). There is a high pitch (~7Khz) resonance with certain types of music. Classical music with lots of strings or brass provokes a faint sound like a wine glass is shimmering in front of each speaker. But it's not super obvious. It's some kind of digital artifact. The FireWire cable I'm using is a little long. Another computer might not have any such problem. But if I listed to the same exact music file on my iPhone, the sound quality is fine. The 48v supplies cause a little 6db whistle at around 10kHz. Sounds like a faint wind blowing in the distance. I have seen posts in forums about this exact problem. And also the headphones out is not super powerful.
Otherwise Traveler in is a very useful piece of equipment. And I put it to work hard everyday as explained above. The CueMIX software is very good. I thought I would care about controling everything from the front panel but I just use CuMIX for everything. Never used the digital effect although sometimes I'll mess with the parametric filters with e-guitar to get different tones.