They are okay for telling the difference between something that is 100 g and something that is 150 g, but accuracy is not all that great. I would suggest using a more accurate balance for chemistry related endeavors, but as Rhodium mentioned, it depends what you need it for. If you just bought a pound of something and want to quickly weigh it, you will get an idea of if you were ripped off or not.
The above post is purely fictional. Any resemblance to "real-life" is purely coincidental.