Today I received my D42 Mamba. First a note that Ibasso made use of DHL, a highly corrupt courier company that will deliver your product for too much money and then charge you additional fees at your door. Ibasso's service was good but I loathe both DHL and FedEx. On to the product.
I do a lot of travelling and I wanted a dac/amp that was small and USB powered to replace my Fiio E17. The E17 is a solid little unit that has served me well for some time now. Compared to the D42 it is slightly more compact, better built and offers more options and flexibility; most of which I've never made use of. This is not to say the D42 is poorly built, it's not, it's just slightly less physically impressive than the Fiio.
What the D42 offers that the E17 does not is an on/off volume knob (with well calibrated resistance). This is a big selling point for me as the Fiio's digital buttons are slower to use and I prefer the ease of the knob. The D42 also possesses better sound quality.
Please note that these are initial impressions and I've only had a day to compare the two. I'm using the D42 out of a PC with USB power and have yet to install a 9V battery. It took a few hours for my ears to adjust but the D42 seems to offer a slightly more holographic soundstage, better instrument separation and both a stronger/tighter bass and treble (a touch more dynamic range). It's not a huge jump over the E17 but it is an improvement.
Remember, early days and my ears not yours... So take it all with a few grains of salt. When I've had more experience with the D42 (and my new CIEMS arrive) I'll post some more impressions.