Being one of "those" guys who actually HAS super singles on my RV I think I'm allowed to speak on them.
Ride - they ride excellent. They are smoother than the duals IMO.
Fuel Economy - I have been very happy with my results here also. There was about a 1/2mpg increase going to the single.
Cost - one single cost the same as two duals, so there was no net cost savings either way
Winter driving - the super single (Michelin X-one) does fantastically in the snow and ice, as well as in the rain. I live in Minnesota and use the RV year round and I've never been afraid to drive in inclement weather. I have driven the duals in the winter as well, and honestly there is not any drawback to the single that I can tell.
I feel there is a lot of fear talk regarding them which is largely unfounded. Do the super single tires look cool? sure, but that was NOT my reason for buying them.
The idea of "limping" anywhere on only one tire on a side is not something I would want to do. I have an RV so I can live in it and if I have a blowout I'm stopping. Front tire blow? I'm not "limping" anywhere other than to the side of the road where I can safely stop.
My issue with duals is how many people fail to check the tire pressure on the inner tire? A friend of mine has a Silverado dually and the inner tire was FLAT. But he just kept on driving all over the place oblivious to that fact. When I first rented a RV with duals the inner tire was down to like 35PSI and the outer was at 105. I feel in this scenario I'm more likely to have a tire failure than a large super single that if it was at 35PSI I would SEE the affect clearly and stop driving vs. letting the outer tire take the load and not "seeing" the low tire.
I know most of us RV'ers check tire pressure religiously, but there are those that don't. Case in point - my Featherlite trailer I tow had 11 month old tires on it and I checked all of them before I left on a 4500 mile trip. about 3800 miles into the trip I lost not one but BOTH tires on one side of the trailer and there was major damage to the trailer due to this.
Anyhow count me as one guy who likes my super singles and I would buy them again if I had to do it all over.