I have the solo Sundowner on my FXDFSE and really like it. I tend to go on really long rides and the stock seat, while comfortable, gave me an achy back after 8 hours. Yeah, it's not a cruiser but seriously, I want to ride for 10 hours without discomfort!
The seat is thicker and pushes you forward and up just a tad bit. The windshield breaks a little lower than it should but I've gotten used to it. The reach on the handlebars is a little bit more comfortable for me. My arms have a more natural bend instead of being mostly straight. I have a slightly greater bend in my knee but the foot placement is still comfortable. I'm 5'9" tall with a long inseam and the stock arrangement was good except the handle bars were a bit "far".
My son borrows my bike from time to time and he really liked the stock seat. He doesn't mind the Sundowner but at 6'7", the extra height and closeness is more apparent to him. He likes the Sundowner but prefers the stock seat for his height.