Generally..no. There's a very, very minimal advantage...if any. They chop certain things 'ok', but many things require a knife anyway.
- Most large objects will have to be cut anyways to fit in which means you'll need a knife and a cutting board just to use the chopper.
- Their blades tend to dull and then they mash, rather than chop.
- It takes about 10 seconds to wash a knife and just as long to disassemble one to go in the dishwasher.
- Its very limited in the objects it does reliably.
Some are better than others, but even the best doesn't even begin to compare with a decent knife. Learning a few knife skills will go much, much longer and ultimately be much easier.
No one can reasonably speculate on how long they'll last - it greatly depends on its use. If you use it daily to chop nuts, don't expect a sharp chopper for as long as you would if you chop lettuce monthly with it.