Buy the best underlay you can find.

We had a slab-leak a couple of years ago and after the plumbers left, we had to recarpet the whole house. We chose a thick pile from HD and the BEST padding we could get (it is a memory foam pad). All I can say is WOW. The padding has kept the carpeting feeling like new and we do have definate traffic patterns. I will probably get really sick of looking at it before it wears out. Also, with huge changes in weather and humidity, the foam keeps the carpet from wide changes in size (you know, that stretch when the humidity get real high or storms a lot, or the super dry air that tries to shrink it).

The carpeting that was replaced had also been from HD. The only reason it was replaced was because the water separated the carpet from its backing. (I was real glad to get rid of it - it was NEVER going to wear out: good padding and it was a Berber).

I have a large area rug from BJ's and I've had it for about 10yrs. Its stud up to dog and kids alike. I've cleaned it a few times but its still going strong. I would recommend putting padding under it, even if you do the area. It helps the nap on the carpet stand up more and allows you to get it cleaner. I'm now getting ready to buy a bigger one but this one will just be cleaned and moved to a different area.

Haven't read through all the replies.... just wanted to add my two cents.

About 7-8 years ago we had Home Depot replace the carpet in our family room. We bought a mid-grade carpet and mid-grade pad. The pad wasn't the best they offered, but it wasn't the worst, either. Well, when the installer arrived, he only had the highest grade pad on his truck, so he said he would install it at the mid-grade price. I was thrilled.

I have plantar fasciitis, and I can definitely tell that room has the best pad. I walk anywhere in the house barefoot, I have heel pain, but when I walk in that room, my pain goes away. It is such a relief.