Based on the Tempur-Pedic model, Sonno Mattresses are topped with a layer of visco-elastic memory foam and are pure polyurethane foam in the middle. Around the outside is a soft layer of CoolMax fabric which "wicks away moisture to keep the body cool."
There are two versions: The Prima is the standard and comes in medium and firm, while the Versa has "two individualized halves" that allows you to have firm on one side and medium on the other side of the bed (if you so desire).
Reviews have been positive so far, and chief selling points are the "better workmanship around the covering material" and the price. DWR's is around $400 less (prices start at $900 for twin). Even better, they are on sale right now.