We added them last week at 10.5 months old. After a longggg stretch of illness and teeth (4 in 2 weeks) and travel we hadn't had her in her crib for like 6 weeks. Then wouldn't have anything to do with it.
Anyways, part of a huge transition back into the crib was adding the bumers someone had given us, that we had no use for.
It helped, she is now in crib for all naps and in the evening.
I'll note she is not a climber so I don't have concerns of her using them to get out of the crib. I lie her down awake, she puts herself to sleep where I put her. She wakes 2hrs later and lies there talking until I come get her. She can sit, crawl, stand, cruise but done none of that in the crib.
We also have a video monitor so should she decide to someday get up and play I can see if she is up to no good. I have no concerns using them at this age. But we didn't use anything but a fitted sheet prior to now.
We got one from Restoration Hardware and it is currently in the crib because 1) It completes the room; 2) Baby won't be in there for a while so why not have it look nice if you don't mind spending the $ on one?; 3) I plan on seeing how DD does in the crib. If she ends up smacking her head or getting stuck, then the bumpers will go in the crib with her.