We use Bumpa bits Shoreedge. Excellent results. The hen adapts in a few days and ours started laying again within a week.
Down sides -not many. Need to be fitted using external circlip pliers -any other way is very difficult. They do wear out with extended fitting -we took one off last night that had been chewed through in three months. The bird cannot preen properly so ends up looking very tatty, especially after the moult.
Haven't used traditional beak rings. They look very cruel to me.
I intended to post about these bits with photos showing the fitting, but although we have taken them we can't resize on our Mac. So no photos until we have resolved that problem.