I'd recommend staying with it. If you're young (i.e. straight out of undergrad), it'll seem like the ~5 years you have left is a long time.

As somebody who spent some time working and then came back, though, it's just a short hop. The Ph.D. will only open doors for you in the future, and the six-year commitment is not an easy thing to do the older you get, with greater lifestyle expectations, family situation, and the general crotechety-ness that comes with time.