(Am fairly certain they aren't great if you are going for audiophile level reproduction - but from a safety perspective, they let you hear your environment - note, I have personally never tried them)
Quality isn't amazing, but it's acceptable and it isn't like you'd be enjoying a high quality audio experience with wind rushing past your ears using normal headphones, anyway.
Also beats getting hit by a car that you didn't hear.
I'm a runner not a cyclist, but I use AfterShokz headphones. The sound quality is suprisingly good, more than adequate for listening to music and podcasts when out :)
For personal safety though, they win hands down. I can hear cars and people easily and I'm much more aware of surroundings. Downside is though that if you are running down a busy road, the sound of cars can override the sound of your audio but for me, that's an acceptable trade off.
Also beats getting hit by a car that you didn't hear.