Helmet mirrors are great but they take 2 or 3 weeks to get used to.
After that you get good at adjusting them, and also learn where to position them so that your normal vision is not blocked much. At first it seems annoying to have it hanging there in the upper left corner in your field of vision, but after a couple of weeks your eyes adjust and just sort of "see through" it. The tiny bit you tilt your head each time you look becomes easy and automatic.
If you have gotten used to checking your car rearview mirror without panicking at taking your eyes off the road- well it's the same thing with the bike helmet mirror. It becomes easy and automatic to check your mirror frequently.
I have gotten two "Third Eye" helmet mirrors for my two different helmets. I wouldn't ever want to ride without them now. I ride a lot, and sometimes I catch myself checking my helmet mirror when I'm not biking at all, but just walking down the street.