Well, I tried to minimize this and stick to the original question, as a seat thread can be as interesting as an oil thread, but since you asked . . .
I've put over 230K miles on Bill (Rocky) Mayer saddles on an LT and three GTs. They are a huge improvement over stock, and his staff does a fine job. Their technique is to use a foam base that is pre-molded into an ergonomic shape (think of those old metal tractor seats, or the butt imprint left after sitting in the sand). These pre-molded bases come in different densities for different weight riders, and are custom fitted based on your measurements and how you sit on the bike.
Honestly, my biggest issue was convincing them that yes, I did have plenty of inseam and yes, I did want to sit very far up and back to get myself in the best ergonomic position (as verified by my chiropractor). I think they get many more requests to lower the seat than to raise it, especially with the taller Beemers. Once we got that sorted, the saddle was very comfortable for long days.
But remember I'm the extreme here. I'm riding multiple 1,000-1,500 mile days, and averaging 45K miles per year and I find that eventually, the padding just starts to break down. I've had my GT seat rebuilt a couple of times in 176K miles, and it's due for another rebuild. So I've been looking at other options.
The Russell Day-Long is based on Bill Mayer's original design, which has two metal wings that support most of your weight, surrounded by high-quality foam. This design is a better fit for riders over 240 lbs or so, as the wings take up most of the weight, leaving the foam for comfort rather than main support. I'm hoping this design will also support me on my big-mileage rides, and hold up better over the long term.
In contrast, Rick Mayer carves out the original foam and fits different densities of foam in different areas to support your weight. In that way, he manages good overall support while minimizing hot spots. I've heard him described with respect as an ass-artist who knows his stuff.
In my experience, Rocky is a quality high-volume production shop with lots of happy customers, but if you fall outside their normal range then you may not be as satisfied.
By all accounts, Rick is an artist and while some riders fault his business sense (mostly poor communication), everyone who has one of his saddles loves it.
And my research into Russell tells me they make a great saddle, with the only complaints being from shorter riders who have a harder time reaching the ground due to the increased saddle height.
Since that's not a problem for me, then Russell is my next choice.
There are other custom seat manufacturers, of course, but since I have little direct experience (either saddle time or trusted friends who own them) with Rich's, Kontour, Cee Bailey's or any of the others, then I have less to say about them.
I will say, in deference to the original question, that there is a huge difference between the custom seat manufacturers as listed above, and the replacement seat vendors such as Corbin and Sargent. Those guys have a one-size-fits-all philosophy, and the only way to see if their one-size fits you is to buy it and try it. No, thanks.
Hope that helps.