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I had exactly this same reservation when this first started testing. Those who haven't been on the subway might not be able to appreciate the sheer volume of people and the speed at which people fly through the turnstiles. As you say, any delay would just slow things to a crawl.

It didn't feel any slower than a successful metrocard swipe to me (and a millions times faster than that series of mis-swipes that inevitably happens as your train's door start closing). The sensor, at least on the turnstiles I've used, is placed a bit low, but it seemed like a pretty fluid motion to wave your phone by it as you move forward. There was no need to hover there, or any pause before it beeps and the turnstile unlocks.

I would prefer to see the sensors placed up where the metrocard slot is, but for now the two systems have to exist side by side. I'd imagine we'll see new turnstiles at some point? But then again, the remnants of token slots have been sitting there plugged up for decades now on some turnstiles...
If I notice a train on the signboard is 1 or 2 minutes away or in the station, I’ll just tap my watch instead of trying to fumble with my wallet and use my MetroCard. It definitely seems faster.
 
Just saw a tweet from the MTA that said it works with transfers! I'm not sure that's all transfers (bus/subway) or what, but it's an encouraging sign.
It works both ways now! Now what the hell is NICE Bus doing? Are they going to stop accepting MetroCards and OMNY entirely now? I dont see any news that NICE is going to get OMNY at all.
 
Roosevelt Island Tramway is MetroCard only for now, too, as is PATH. I honestly don’t think the MTA has reached out to the non-MTA agencies that also support MetroCard. The Tramway will likely add OMNY support, they already support unlimited MetroCards, unlike the Port Authority or NICE.

Of course, NICE doesn’t exactly like to play nice (see what I did there) with the MTA, so they may not ever accept OMNY. Actually, can I get on my soapbox for a moment and complain about how Suffolk County’s cashless system is Android only? Thankfully I live in the city and not Suffolk County, but that makes it really hard to visit Suffolk County as a tourist. If you’re not an Android user, you’re stuck paying exact fare, which is legitimately frustrating. Oh, you need to go to the ATM? Well, you’ll have to wait another half hour for the next bus…
 
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