I'm using the Ack!Dak 1.0 with a Cardas Lightning 15 digital interconnect. I've seriously considered getting the Ack!Dak 2.0 - from all I've read its a very noticable improvement over what I'd already consider a very musical DAC. My main reservation is that I need a DAC that supports both RCA and Toslink inputs (and if possible BNC and balanced too for future upgrade possibility).
But, you're probably more interested in what the Ack!Dak brings to the table in terms of redbook over the SCD-777ES. I think the SCD-777ES is excellent in SACD mode - tonally adept, excellent soundstage and positioning of instruments with excellent dynamics and extension at both ends. Unfortunately, I don't find this in redbook mode. In my system I find the stock SCD-777ES in redbook mode almost 2-D in terms of soundstage, fairly bright on the upper end and overall not very musically enjoyable. What the Ack!Dak brings in my system is marked improvements in these areas. Overally the Ack!Dak 1.0 is very musically adept. Areas I'd like to see improvements in the Ack!Dak 1.0 are an even fuller midrange, more extension at both frequency spectrums, better dynamics and a bit more soundstage depth. I believe the Ack!Dak 1.0 is a superb DAC overall, especially for the price - what I'm pointing out as areas for improvement are (a) based on personal taste/bias (b) areas I've heard improvements in when listening to much, much more expensive digital setups (e.g., BAT VK-D5SE, Linn CD-12, ML 360). So, do take this in context. I understand from what I've read that the Ack!Dak 2.0 goes a significant ways towards improving in some of these areas - so you're probably on the right path to consider an Ack!Dak 2.0. And, I honestly wouldn't be surprised if after owning it for a while you consider it as the end of your redbook upgrade path.
I hope this helps.
All the best,