I do the same thing, prime is great. One thing that does make me feel a bit guilty about it though is the amount of packaging wasted when I'm constantly ordering all of these things from Amazon. I usually have a couple boxes stacked up every week, even if it is just a few small items ordered sporadically throughout the month they usually ship in a box about 8x10"... That's a lot of cardboard being used now compared to going to the store... At least that's what is seems like, I haven't actually measured the amount of gasoline and plastic or paper bags used when buying locally.
Regardless though, Prime still seems like a great business model if it is working for Amazon. Every time I have to order something online that Amazon doesn't carry I always feel like I'm going back to the stone age having to pay $20 for 4 day shipping.