The gear advantage of a mid-drive rocks but I was quite surprised that I chewed through a rear cassette in under a year / about 1k miles -- a 250W motor + heavy cargo bike + kid passenger + a large and pretty strong rider exert an awful lot of force. And with the assist, I almost never use the larger cogs, so all the wear gets concentrated. (This compares to many, many thousand miles on my road bike.)
Still love the thing, though.