Magnets are one of the cheapest and most fun toys available. They do however come with the slight consequence of being highly deadly, so you should never give them to small children or let pets near them. If you swallow a single magnet, the main risk will just be it being a choking hazard. When you swallow a pair ... you run the risk of them connecting through your intestinal walls causing serious damage.
When it comes to magnets, the more of them that are connected, the stronger they become. The stronger a magnet is, the faster and harder it will pull any magnetic metals or magnets. If you keep on playing with strong magnets, your fingers will eventually get pinched or worse.
All in all, they're worth the risk considering how fun they can be.