I bought a 12-oz Tall for $4.15.
I don't believe they asked me if I wanted whipped cream but my drink came without it.
While I wouldn't say it tastes like mascarpone, the drink does feel tiramisu-ish and wasn't terribly sweet for a Starbucks flavored drink (of course, it may vary depending on who's making your drink but you can always ask them to put more or less syrup to taste). It's possible that the lack of whipped cream had something to do with the moderate sweetness. Like a lot of flavored coffee drinks, there's not a lot of coffee flavor to it.
Overall, I found Starbucks' Tiramisu Latte pretty enjoyable but for the same price, I'd rather have the actual dessert.
Nutritional Info - Starbucks Tiramisu Latte - Tall with Whole Milk (12 fl oz)
Calories - 220 (from Fat - 120)
Fat - 8g (Saturated Fat - 5g)
Sodium - 110mg
Carbs - 28g (Sugar - 27g)
Protein - 8g
Caffeine - 75mg