Well I tried to use this for the first time at DEN and it didn't go too well.
I checked in on the mobile app the prior day, and selected 2 bags to check.

First kiosk, I scanned my boarding pass and the screen changed to "KIOSK CLOSED". LOL, ok next kiosk. Boarding pass wouldn't scan. Tried the one next to it, wouldn't scan. OK no problem, entered my MP number and it came up and said "printing baggage tags". First one printed, then I waited....and waited...and... no second tag printed. Tagged one bag, brought it up to the belt and explained my situation. Agent sent over a contractor in a blue vest to print me another boarding pass. We walked over to the kiosk and she added a bag to my res, and the machine wanted to charge me $150 (!!) - This made no sense on a number of levels since I was on a First class award ticket. Fortunately she had some sort of override card which brought up a list of reasons to remove the charge, ranging from various elite statuses, intnl itinerary, etc etc. She just said "choose one." LOL, ok, I chose "Customer Already Paid" .

On top of this the conveyor belts were down at DEN, so I just had to put my luggage in a pile and trust (hope?) that somehow they would make it to my plane.

I checked the app after clearing security and noticed I now had 3 checked bags attached to my itinerary. Only 2 actually tracked being loaded onto the plane, etc.
In the end, both bags made it to my destination. But the process certainly did not save me any time, and required 2 agents to help me, rather than one, so didn't save United either.

Just one data point, hopefully an outlier.