That's only partially true.

The ones you can find at Target and Walmart work fine -- the headpiece is often a bit bigger than the more expensive name brand ones, and the magnets tend to be weaker so you may have to mount the magnet to get closer to the sensor, but they work fine and are often around $10 or less. Realibility wise, they're as good as anything else -- wires tend to get snagged and the like rather than having anything else break.

Personally, I pick up cyclocomputers at the local bike swap meet, Frankenbike. I'm not sure why there's so many, but I often find good quality name brand cyclocomputers new in the box there for $5-$10. So every one of my kid's bikes has one, and I've got a few spares if needed too.