Yeah, thats normally my experience also, they either work or they don't.
The manager of the parts store said that he does see weak starters though too. He knows that I don't buy parts like this from him, I use a mail order German specialist, so I don't think he was saying this to get a sale.
I made sure that the connections at the battery end were good and clean, but I didn't check the starter end. I'll have to do a voltage drop test too before I go spending money on a new starter.