I have not yet had the luxury of participating in the TSA Pre-Check line but the Sky Priority has been hit or miss for me on a couple of occasions now.

On my most recent trip out of Logan, there were two separate lines. The first was for the part where you give your boarding pass and identification to the TSA agent who greenlights you to move forward. The second line was for the examination itself and this was moving painfully slow on the particular day I went. So in this case, the Sky Priority did nothing for me. I ended up waiting 15~ minutes anyways.