This is a really fun thread. These are the things I use to open my letters.
Back in the 70's when my father took over a small doctor's practice in Glassport, PA, the retiring doctor gave the letter opener to my mother. It's severely dulled, a little bent at the tip, but I still use it.
The Xacto knife comes in handy with those thicker envelope papers. I usually start at the top, but if there's resistance from a liner, I open the letter on the side. It's not easy to open up those flaps with beautiful liners. I'm sure there are many guests who tear into wedding invites without realizing the care that went into lining the envelope.