After about 90 days, the rate spontaneously increased from Prime + 0.5% to Prime + 1.0%. She claimed it was a teaser rate - sadly for the bank, our signed contract said nothing of the sort. I fought it, and so long as I'm staff, it'll be Prime + 0.5%; I believe it goes to Prime + 3 or 4 if I'm no longer an employee.
Generally speaking, unsecured LOC rates are likely commensurate with the risk entailed (e.g. credit cards (unsecured credit) have astronomical rates, I imagine due to a higher probability of default).