Should I buy a mechanical chopper?
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.