Mobile Passport Control (MPC) at IAD experience?
 

Has anyone used used MPC at IAD recently? How was the experience? Did it actually save you time? Traveling with a baby who doesn’t have GE and it’s really not worth it for us to sign her up given how infrequently we travel internationally since she’s able to piggyback off our precheck.

Depends on how you define recently. I did in December 2021; my GE status was unclear since it had been extended due to a lack of appointments from COVID. I kept trying to get an appointment and nothing made it clear whether I still had GE or not.
Flying on EK I went ahead and tried to enter through the GE kiosks and they said "nope, you expired". I downloaded the mobile passport app, filled everything out and got through in probably an extra 5 minutes. Then went and did my GE extension in IAD customs.
EK of course arrives when there aren't as many other international arrivals.

I can't recall seeing much of a line for MPC in subsequent trips through IAD.

It’s still possible to generate MPC codes for IAD arrivals. But the days of MPC being a reliable time saver to get ahead of the crowds and/or minimize additional questioning and passport checking time became history when CBP got into the business of wanting to push Airside Mobile out of the scene and to ramp up Simplified (Biometric) Arrival. It’s not unusual for CBP employees even at a place like IAD to sometimes not know or care what to do with MPC users other than to just do the same thing as for the rest of the returning American masses.

Is that still the case now that the only MPC app on the market is CBP’s own?

The two initial MPC-enabling apps which I had were knocked off by CBP. The only MPC-enabling app which I’ve used in the last several quarters has been CBP’s own app.