Bottom line up front: The agent at Matlen Silver ghosted me instead of delivering presumably negative feedback. There were intermediate conversations that seemed to string me along. (See below).
This is more about the feedback from the client firm, and the coordination with Matlen Silver as the contract agency go-between.
The interview process I would describe as "old school", and I would suspect it would not be indicative of other interviews at this firm, which is one of the reasons I'm omitting their name.
The questions were about C++ features, asked verbally, quiz show style. They were quite reasonable for people with C++ experience.
The final, on-site was similar, also with discussions about prior work experience.
Feedback through Matlen was spotty at best. In almost all cases, I needed to reach out to request updates. There were several cases where I had to ping them twice. The actual updates were never "pass/fail", but rather, "They are having internal budget realignments" or other such business related arcana typical of delays at huge corporations. I never could tell who, if anybody, in the food chain may have been stringing me along.
In the end, I never received a final answer.
In total, I would advise caution when using them, if only for the sake of the ultimately unprofessional communication style. If the job they're offering is a great fit, there's no reason to not apply, just be prepared for shenanigans.