I have been using them for awhile............most of them have been very sharp and have lasted longer than the more expensive kind; I have had no problem with them fitting, and I have 3 different rotary cutters (different brands). Out of all the ones I have purchased, I have only had 2 that were not very sharp to begin with; at that price, I think they are a good buy and seem to work well........maybe, If you got a dull one, take it back and request a replacement; where are they made?