I picked one up for $4.99.
The new sirloin patty is pretty big and meaty. The flavor seems more beefy and less seasoned than I remember the Angus being, which is different but still nice. My particular patty sported a nice browning on both sides and was moist with a fairly coarse grind. You might get a fine crust if you manage to get one fresh off the grill.
Perhaps it was the more simple composition or maybe it was buoyed by the other ingredients, but I could actually taste the white cheddar rather than the vague creamy impression I'm normally left with. It could just be that there's two slices though.
Overall, I really enjoyed McDonald's Steakhouse Sirloin Third Pounder Burger. I especially liked the meatiness and peppercorn sauce.
Nutritional Info - McDonald's Steakhouse Sirloin Third Pound Burger (276g)
Calories - 730 (from Fat - 320)
Fat - 36g (Saturated Fat - 16g)
Sodium - 1560mg
Carbs - 62g (Sugar - 10g)
Protein - 39g