I've been self-employed my entire life, so I have had a real a$$hole for a boss (me!). Because of that, no matter what kind of work I'm doing, I always do the best job I can. Gotta make my boss happy! My customers appreciate it (usually).
As an Employer, I appreciate loyalty in my employees. Sure, if they get a better job offer, I don't hold it against them if they take the job - they have to look out for themselves.
But beware - the grass may look greener on the other side of the fence, but you haven't seen your neighbor's water bill, either!