It's easy to burn out a motor by dropping unopened envelopes and junk mail into the shredder, which may contain more paper than the shredder can handle.
If you're looking for a large shredder, make sure that the bin has plenty of space too or you will end up emptying it more often than you'd like.
Pay attention to the amount of paper you are shredding - all shredders only shred a certain quantity of paper at a time.