When they are new an in adjustment, they are great. If they are worn out and out of adjustment you will hate them. When using them you need to keep the sickle in very good condition. You do not have the top of the guard to aid cutting. The stub nose guards cut more like a pair of scissors. So the clearance needs to be kept close or the crop will hair pin around the guard and not get cut. Kind of like an old pair of scissors that are set too wide.
If you mow a lot of hay in damp conditions then they would be worth the investment. If you have mainly grass hay and mow when it is good and dry then they would not be that much better than a good conventional setup.
So if you are buying a used machine with them really check on how worn the guards are. The are not cheap to put in a full set.