I have one of those and I poach an egg for my son every morning. They work great, but over time the latch wears or warps and it can pop open and blow egg shreds all over the inside of the microwave.
Until I can remember where I got it and replace it, I have been using a rubber band to keep it shut.
The soft silicone cups that you float in boiling water produce a prettier and more consistent egg, but take 10 minutes instead of 1.