A friend of mine has one. He is happy. It represents excellent value for a touring bike. He is a moderate-weight rider. I think the suspension seatpost helps a lot. He rode a fleche with me back in November with it. We did 376km in 24 hours. He was comfortable and happy, although he is looking for a different saddle.
There are people who might argue against an aluminium touring frame on the basis of harshness of ride and the remote possibility of breaking something. I am not sure how the rear derailleur hanger is set up on these, but that would be the only vulnerable area that I could think of. My mate uses his for commuting as well as touring and other long rides.
Brakes are good, fork is Cro-Mo which is good and it is eyeleted. Three water bottle locations. Shifters and gear set-up is fine. The only thing you might have to watch is whether the LBS has cut the steerer tube too low and the handlebars are way below the seat level for your adjustment. If it is, you should get them to swap in another Cro Mo fork with a higher steer tube and spacers for the stem.
Frankly, for a ride across the United States like you are planning, dreamy, it would be pretty good value. I'd say go for it. The sooner you have a bike in your hands, the sooner you can get out there, get used to it, work out any issues with fit and your knees, and start having lot and lots of fun.
PS: You will get some posters on these forums who might say Mongoose is just a *mart brand. That may be true at lower levels, but I understand the importer in Australia for Mongoose made a special effort to specify this bike to suit the touring market. The frame is likely produced by a factory that already makes moderate-level frames for better known brands, anyway. I considered buying one at some stage two years ago, but was happy with the way my Fuji was turning out.