Trust me, my family used one of those things and they suck! Like other people mentioned you don't have sharp blades after a while. If you have more solid vegetables like carrots it's way too easy that the carrots get stuck and bend the blades. after that you can try to pull it out or cut the stupid green plastic at the top and search for little plasticparts between your vegetables. A friend of mine bought one of those cheap around christmas and what should i tell: same problems.
Buy some good knives