I'm a big Day Long fan. I got mine a couple of months ago and never looked back. It's very, very comfortable and I really like how it makes me feel more like I'm a "part of the bike" (e.g. no more sliding forward or sideways). I had put an Isuzu head rest on the stock seat for back support and it helped quite a bit, but the backrest that Day Long sells is much sturdier. These seats are not cheap, but worth every cent in my opinion.
The only question I would have for you is whether the Day Long would end up being too tall for you. You say you're 5'11" and your legs just fit the bike. Maybe that means you have a long inseam, which would be good with the Day Long. I think their seat ends up being an inch or so taller than the stock seat due to the spring they install in the seat to create the side support and the extra padding they use. However, they also shape the seat so it's narrower toward the front where the seat "horn" is. This enable you to slide forward a if you need to in order to flat foot it at a stop light.
Check out the pictures of my seat in the gallery under Gary Smith #24.
Gary