I've sold 3-letter .US Domains for $200-$1400 this year, and am currently negotiating two more possible sales, including one of my favorites - Dev.us. My previous sales were quality letters, and a couple good acronyms. So yes, they are definitely good investments in my opinion.
Good quality 3-letter domains in ANY of the main extensions are a safe bet.
From my view, the biggest difference in .US right now, is the less desirable letters are not selling for much. This can be looked at two different ways. A) Don't bother with those type letters (Q,Z,X etc.) if you are looking for quick turnaround profit. B) On the other hand, there is an opportunity to grab these type domains cheap, and make a stash for the future, with the expectation they will rise in value just like we've seen happen with other extensions. Even the less desirable letters sell for nice prices in those cases, and .US might not be far behind. Only time will tell for sure.