The magnet part is strong and I use them at work to attach pictures notes etc. to my overhead cabinets. The colors are not that great and the plastic part is cheap but they do the job.
Keep a variety of sizes attached to the chalkboard for quick access. Great for single sheets of chart paper or student work - easier than clip type magnets. Students like the colors.
Having just moved into a new office that has metal walls we wanted magnets to affix items we'd normally attach with thumbtacks or nails. The strength of the magnets makes them only marginally useful