I started looking into them and they offer very competitive pricing and compelling products. I have not tested any (would love to try out their TCU tune) so I cannot comment on support or quality.
However they offer dyno tuning, bench tuning, and importantly TCU tuning. TCU tuning drops the 1/4 mile time on the m278 and m157 CLS and E class (w212 and c218) by .3-.4 seconds which is a huge boost.
Chris Mattox mentioned to me he actually prefers going with the TCU tune first as there is no need for spark plug replacement. That being said, it likely isn't a bad idea to do a transmission fluid service before TCU tuning.
Both for good measure and because hitting the Torque limiter looks/feels a lot like slipping out of fifth gear.
Check out solo motorsports TCU tune here, for an incredible $1500, plus a refundable core laptop fee of $1500 if you aren't on site.