I currently have the black Speck (plastic shell, silicon lining) case which I purchased at the AT&T store when I got my Lumia, which brought the weight up to 219 grams (7.725 ounces). I bought it because I knew I would be dropping this phone eventually, and because of it's weight, a thin, light, gel case will most likely not prevent dents. I've gotten used to the weight pretty quickly. When I first bought it, yeah, it felt heavy, but now, it feels "substantial".
Judging from the amount of microscratches on the case, I think it was a good purchase. The screen is naked, though, and still immaculate. Gorilla glass does the trick.