We took it going out on our first trip to Owl's Head a couple of weeks ago, so my perspective is probably one of the less experienced ones you'll get, but here goes - it saves one mile each way, but no time. (Based on coming back on the trails.) It does, however, cut out three major stream crossings, which depending on conditions may be very valuable. (It wasn't when we went, which is why we came back on the trails.)
There was a small path at the BP end, a more pronounced path at the Lincoln Brook end, and not much in the middle. Going to Owl's Head, you don't have to be too precise because you hit the Lincoln Brook trail broadsides. Coming back, though, you need to be more precise because you'd be hitting the Black Pond trail end-on, and you certainly want to be on the correct side of Black Pond. I had a GPS track for it which helped a lot.
My advice? If I were doing it again, and the water levels were low, I wouldn't bother. Easier to take the trails and put it on cruise control.