I have both, I love the adjustable one for long trips. It lets you change things up a bit. The down side is the size of the mount. With a stock street/road glide seat the mount well hit you on the tail bone. With an ultra seat it is not an issue.

 

Yes, very happy with the HD fixed backrest.

 

It seems many are happy with the fixed HD backrest so tomorrow I’m going to try to pick one up.

 

I have an '08 FLHT with a Sundowner seat. I bought a cheapie backrest on Amazon for about 40 bucks. Didn't expect much, but it does what it's supposed to do. So, no, a proprietary (Harley brand) backrest is NOT required. I understand that the mustang rest is also compatible.