My thoughts on the iDo (posted online elsewhere, and much of this copied here):
I do really like the iDo and use it almost every day. The iDo's primary job may be to replace the DAC and amp in the iPod with a nicer DAC/amp for desktop listening, but in my case I also use the iDo feeding the iPod > coax digital into my nicer desktop DAC and amp. It's very useful for when I don't have my Macbook plugged in or turned on but I want to listen to my favorite playlists without a loss in sound quality. I basically get the same sound quality with my iPod > iDo S/PDIF coax digital output > DAC > amp as I would if I was using my Macbook > USB DAC > amp. It's like using a CD player's S/PDIF output into your DAC, but with playlists and not having to swap discs every 40-50 minutes!
The iDo headphone jack is more than adequate for casual music listening and sounds as good as my uDAC-2 headphone out, beating my Icon desktop DAC/amp for clarity and depth. It doesn't have a lot of juice for the most demanding headphones, but with most of them it gets the job done better than my iPod or iPhone by itself (6v power supply). Granted, it does sound better when feeding it's RCA outputs into a better amp, or feeding the coax output into a better DAC + amp. I routinely feed the RCA outputs into my Pico Slim or RSA SR-71b balanced amp to listen to my custom IEM at home. Just like with my uDAC-2, I find the iDo's RCA line-out sound quality is better than the headphone jack, and it rivals the RCA outputs of my more costly Pico DAC-only, but without having to boot up the computer to get that sound quality from a USB DAC.
Sure, it's more expensive than something like the Pure Audio i-20 dock, but when you consider vs the Wadia digital dock that costs a lot more and doesn't include an amp or upgraded DAC, then the $249 price becomes much more attractive. For those complaining that it still needs to be plugged into the wall, you can go spend $600 for a Cypher Labs battery operated digital dock, plus add an amp to that. I'll concede that even a budget $180 desktop amp like the HiFiMan EF2A or Travagans Red sounds a little better than the iDo's headphone output, but to me the iDo is all about being a great source, extracting lossless digital music from the iPod, doing a better job converting to analog, or sending pure digital output to an even better DAC than the one included. Having a decent headphone output is just icing on the cake for me.
I especially appreciate being able to use the Apple remote to control the volume and skip tracks, and being able to use it with my iPad as well (there is no funny shaped cradle that wont fit an iPad). I like that if I don't want to use the remote I can have my iDevice in my lap with the sync cable running to the iDo, instead of the iDevice being stuck on a dock built into the device. The only thing I'd like to see added is the ability to also act as a USB DAC for a computer (since it has a USB port on the back for the iPod sync cable), or maybe having pass-through video via the line out dock so you can connect the output to a TV. I did once ask about the lack of 1/4" jack, and Jason Lim told me that they simply could not squeeze a 1/4" jack onto the PCB without a major re-design.
Overall, I would hate to be without my iDo. When I listen in my bedroom I often like to use an iPod, iPad or iPhone and don't want to tie up my Macbook as a source. I can quickly plug in and listen to music without much fuss, using IEM or full size phones with a 1/8" adapter.