The back cover I guess is ok, I think it's durable, though it does feel like cheap plastic. Even with the texture on the back the sides are still smooth, so people saying about good grip, I'd like to see how you hold it that that texture matters ;p
The usb cover - it's useless (read as "retarded"). First off all, let's talk about the position of the plug, it's on top, which mean there will be no dock or anything related for this phone. Then this stupid cover, what does it protect it from? dust? gee how many phones anyone had with no such cover and has problem with dust? And it's not like this plug get used once a week, it's used at least twice a day!
I think the reason for the plug, is because many if not most people carrying the phone in the pocket and it lays there upside down, which means more chances something get jammed in that hole.