We have a monk in our oldest campaign(since a little after release) we are level 17. He has had mobility for quite some time, maybe 4 on. It has always been useful for him. He is a Elemental monk so he uses tons of ki dropping spells on people and stunning, doesn't really want to use it to have to disengage. Also using ki for that uses your bonus action too, so no Flurry. We usually get either 1 big thing or umpteen small ones that still hit really hard so getting Opp atks for him isn't really an option, his ac is good but he still goes down quickly. Basically having mobile lets him do more damage because he doesn't have to use his ki to stay alive. Stunning is good and does take their oa, however it isn't a guarantee and I've seen him blow 2-3 ki trying to stun one guy and failing. Mobile is no-questions-asked, no save, no action economy and no ki, it also gives you 10' of movement, it's a great feat, not mandatory of course but great.
As far as how he survived at low levels, it was a lot of hit-and-run, he did go down a lot but we have a decent group for healing so we haven't let him die yet.