I used to do this when using a film camera years ago and did actually get some great shots but that was with a 70-210 lens so most of them never knew they'd been shot. The zoom lense is great for that and I used to take some of buskers as they were always happy to pose and I'd chuck in a few quid after as well
The only problem I ever had was when one ugly looking bloke saw me taking a picture of him and he swaggered over with his mates asking what I was doing, I simply winked and replied it was cos he had such a nice ass!!! His mates started taking the mickey so much he turned bright red and walked away whilst I was blowing him kisses!!!!