You're right. There are some cycle computers that use ceramic magnets, which are not so strong. Most these days use neodymium, like mine. They're very strong. Even if you have the weak type, you can add a Neodymium magnet to it. It'll stick by magnetism. This is very useful if you can't get a cycle computer sensor close enough to the magnet, like when you put it on the back wheel. The increased magnetism allows a bigger gap. You can tell what type of magnet you have by the colour. Neodymium is shiny silver. Ceramic is grey. The ones below are N52, which is about as strong as you can get.
http://www.ebay.co.uk/itm/10-Very-Strong-Neodymium-Disc-Magnets-10x2-mm-N52-craft-reborn-10mm-dia-x-2mm-/161316028750?pt=LH_DefaultDomain_3&hash=item258f2f414e
These magnets are really useful for holding things in place. Uses are only limited by your imagination.
If Wissy can try one of mine, then we have the definitive answer.