They are pretty popular but unless its some sort of premium keyword or city/state in the us it probably won't be worth much..If you plan to develop thats a whole different story.
there are many people owning .us domains in a list of my domain priorities .us comes 4th after com, net, org. tell del.icio.us that they've got a stupid domain lol
I think it may take some time here. People are still caught up in the .com in the US it seems but I have a few decent .us names. If you find some killer keywords, they are definitely worth grabbing I think.
In the US, people are a lot more accustomed to .com over .us. Plus they have stupid restrictions that are holding the extention back unlike the .co.uk & .de where anyone in the world can register them.
I've bought some .us names. I think they will continue to go up in value but if you were to get them appraised right now I doubt they will be worth near as much as a .com or .net.