I used the Cheq app to make UPI payments via smartphone / QR code, during a holiday in India in July. The app listed locations for completing the Know Your Customer (KYC) process - I did this at Transcorp in Bengaluru. It was worth the effort - it is far more convenient than cash in most cases. Most vendors kept little change, and even restaurants were not keen on receiving a 500R note for a 200R meal. Similarly for museums, art galleries, historic buildings. Auto-rickshaw drivers were happy with cash, and had a reasonable amount of change. I did find a couple of vendors who weren't registered as merchants for UPI, and I could only transfer to merchants, not other users. For travellers I'd suggest setting up UPI beforehand if possible; as mentioned you can do it as you fly in (Thomas Cook offers this service in the airport), but the KYC might take a while.

(Side-tracking from payments to SIMs : ) Several historic buildings had an online-only ticket process with a QR code displayed outside. The user experience of the websites was variable (navigating the forms and input widgets on a phone while standing in the street could be trying), and they required a lot of information, eg. Passport number and sometimes home address, usually including authentication via one time password sent to mobile phone number and possibly they only work for Indian SIMs. Those sites accepted credit card payment, including international. I couldn't count the number of times I gave my phone number, for all sorts of purposes, e.g. when checking in my phone at a temple, I was photographed and gave my number - simple id and method for finding the phone among all the others (when I realised that I switched the phone back on). And for all of these a local phone number is required, so pick up a local SIM at the airport or in the city. I did both, and I think the airport process was quicker. They do require full id for SIMs, including a local address, e.g. a hotel. A 28-day SIM cost only 299 rupees so I got a second one for increased coverage and capacity, because the networks are over-subscribed. (That's less than the daily rate at home). A dual-SIM phone makes that easy, and I could also run with one local SIM and my home SIM to receive SMSs. I was able to download maps and apps while travelling on buses and trains, and use tracking apps - which helped with knowing when a bus was approaching my stop.