Originally Posted by Weekend Away

Yes I have, with mixed results. With MU, I took my gold card with me on all of my flights and put my skymiles number on the reservation. I was able to board with SkyPriority on all of my flights. (Occasionally I had to explain things, but they let me through) I used the VIP lanes at PVG using my card. I used the SkyPriority check-in counters at every airport. A few times I even managed to get SkyPriority on my boarding pass but with my skymiles number on the BP.

When I fly Delta I just use the app and don't check luggage on domestic flights, so I can't speak to using their check-in counters. I can, however, use the SkyPriority lanes at ATL. Though they don't often check my boarding pass. I've only tried once to board SkyPriority with DL; I'm usually late to the gate. The one time I was on time I was told to wait until zone one, even though I had my card with me. Not the end of the world, as I'm pretty sure that is the rule.

As for seat assignments. You can enter you FBN on the reservation at booking and choose priority seats, ie exit row and front of cabin for free. Then switch back to your Skymiles number without losing your seat selection. (Make sure to do this prior to printing a boarding pass with your FBN as you won't be able to switch, only gate agent can, and then they were only able to remove it and not add my SN.)

Overall, I'm still able to get everything out of it.