I really wanted to give 5 stars because I love the small form factor and the ink wipes off cleanly regardless of which color I'm using. However there are some issues that make me hesitant to keep using them:
The plastic caps are really thin and flimsy. Within the first 30 minutes of use, one of the clips had already broken off, and a few days later the cap itself was cracked, meaning it would no longer close properly.
The ink takes a while to dry and tends to rub off fairly easily, even after drying. I've been trying to use them on reusable flash cards to help my kid with her multiplication tables, and after a few shuffles of the cards, the numbers are fading. I also tried using them for prototyping a card game, and put the cards in sleeves to prevent my fingers from rubbing the ink off, but then they ended up leaving residue on the insides of the card sleeves.
The black ink pens that are sold elsewhere on this site seem to be more sturdily constructed and the ink does not rub off as quickly. I wish the colored pens were more like the black ones as those seem to work better!