On public transit and airplanes, definitely. I used to get colds all the time when traveling; I don't anymore. I think part of that is down to improvements in air filtration, but part of it is masking.
In some shops, but not all. Mostly in shops that have a lot of people in at once, and shops that request it. I don't mask if I'm going to the corner store to pick up some beer; I do mask at the grocery store that has a sign requesting it.
Never for outdoor events. This might be different if I lived in a different city; a friend of mine who lives in the LA area does mask for crowded outdoor events.
Also, doctor's offices and a few other places require it.
(Curious observation: On transit, I'm not usually the only person with a mask on, but I'm often the only non-Asian person with a mask on.)