I have been on a 16 year quest to find a good seat for my Interstate. I can't ride much more than 50-60 miles before having to stop for a few minutes. I have low back problems and my hamstring muscles tighten up causing me to take breaks.
First got the Mustang because I needed a backrest. (Didn't want to cut into the OEM for a Utopia.)
Wasn't happy with the Mustang. The seams on the seat cut into the back of my legs and, under sudden braking, I would slide up on the tank.
Got an Ultimate Big Boy with backrest. Current set-up. The backrest is a joke. Mine is the earliest version, square post and adjust-ability is Very limited besides the backrest pad itself is small.
Been tossing around the idea of getting a Utopia backrest for my OEM.

Not sure that will be the answer.
When I picked up my Std in 2004, Ultimate came out with the King. Bought one while at IZ in 2004. The bike came with a Utopia so that is the backrest on that bike.
When ever switching bikes, I thought the Std was a bit more comfy. Last night I put the King and Utopia on my I/S and took a 50 mile ride. Strange, it didn't seem to be any better than the Big Boy. Not enough to justify over $600. for a King and backrest.
Next option would be a Day Long. Their web site is very hard to understand, but as far as I can tell, if I send my OEM seat and they just do the driver area and not the passenger seat the cost looks comparable to a King w/ backrest.
The site is so hard to understand, I guess a call is in order.
Those of you with a Day Long, what other seats did you try?