Main issues other reviewers mention:
1. The velcro attachments aren't strong enough so you risk your controller flying across the room and smashing (as well as possibly smashing something else in the process)
2. They are lightweight so feel imbalanced. This is also where I think they are overpriced for some basic plastic inserts
To solve the issues:
1. A stack of about 50 x 1p coins fits nicely inside each handle. Taping them up and gluing them in leaves you with a well balanced handle that has real substance and feels so much better
2. I found and bought some re-usable cable ties. The 7.6mm width ones fit perfectly in the hole and cost about 10p each. Using these you get a really strong secure hold without modifying anything. Before each use I test they are holding strong (none have failed yet). You can then release them and get the controller back very quickly and easily.
Overall I'm really really pleased to have these. I play Beat Saber a lot and these make it even more enjoyable.