When it comes to leaving one job for another, I'm often surprised when people tell me they are worried about letting down their current employer, or appearing disloyal, if they leave for the new job.

IMO, that's a pretty stupid way to look at it. Even the most loyal-appearing employers will cut you loose the day they decide it is the economically superior decision.

That doesn't mean there isn't value in staying where you are, or value in an existing employer-employee relationship. But don't stay at a job because of "loyalty." Or at least be aware that it won't be reciprocated.