Echoing the comments about PreFlight parking near Logan. We've used it several times when the cost is better than a taxi or other livery. I agree it is tricky to get to, and we budget 20min from car to airport for "just in case" happenstance.
The preflight is a fenced lot, and the shuttle from the airport will pick up and drop you off at your car. Although I don't drop my guard, I do feel rather safe at the lot. We've used it very late night and mid-day, I've felt the same sense of safety. Even after a major snow storm, our car was shoveled off in advance. Their frequent parker/loyalty parking was helpful for us, we stayed "for free" one time using up loyalty points.
Since you're north of boston, you might consider the Woburn park and fly. I haven't used the park and fly, so I can't comment on the logistics, cost or efficiency. Sean's mom uses them often and likes it.
Safe travels! -Krista