Personally I've had no problems with my unit, the previous owner did digitalized some vinyls and was thoroughly impressed by the quality of the files.
I haven't tried yet a turntable, because I don't even own one, and I've not done ADC conversion but I know from the previous owner that it is functional and well, but all the rest satisfies me about it, the DAC, the headamp and the preamp. Yet like ursdiego explained, it's good for me because I'm coming from using just my laptop, and I didn't have money for more.
Very much all the reviews I've read about the GT40 were positive on all aspects but the phonostage which sounded lacking to some of the reviewers. Sound quality-wise, the ADL GT40 beaten single-handedly the DACmagic plus according to a reviewer, and the NuForce Icon HDP according to another. The DACmagic+ is 600 dollars (same as GT40) and Icon HDP 450$.
The GT40 is fully silent, with black backgrounds, so I like that... but the potentiometer could be better in my opinion (be smoother and allow lower volumes listening).
I never had compatibility issue, it's plug and play. I have had 3-4 bugs in the year I've owned it but all it took was a power-off + power-on to fix the unit.
Also, it comes with a linear external power supply already, and not switching! I can't copy text on ADL-AV.com but here's the last question of the FAQ for the GT40, look at the last paragraph of the answer:
Edited by devouringone3 - 6/14/12 at 1:21pm