Have these in our 3 seasons playroom. A few observations:
Fully expect the insert letters to be pulled out and rarely returned to their spots. At least this is the case with our 4 year old and his three cousins aged 5 - 8.
These are pretty thin, 1/4" at most and extremely lightweight. If wind can get at them, they will blow around.
Our cat thinks these are excellent to chew upon or dig his claws into. He hasn't eaten any pieces.
It took 3 sets to do most of a 10' x 10' room.
If I had it all to do over again, I'd get larger, colorful, non-alphabet cut-out tiles.
In summary, they are a decent product, depending on your needs and your kids temperaments.
: Tiny plastic superheroes, you say?