My only HK experience is reading about them on the "net" and handling them in the gun store and at the gun shows.
The USP 45 has a great reputation for accuracy, reliability, and durability. The DA trigger on the DA/SA models is often commented on as being "brutal" as far as DA triggers go. Depending on the model, it can be carried in Condition 1 (cocked and locked) and operated single action, though in my limited handling of them, the thumb safety is not as natural as the one on a 1911. Hilton Yam has tested an HK45 (different gun, admittedly), and has ended up with the safety/decocker positioned such that the trigger locked up in a "no man's land" where it wasn't working. Yam is a long time 1911 shooter that rides the thumb safety, and while it may be a problem for a 1911 shooter, somebody not trained to do that may not have the problem. I don't like the safety/decocker location and operation (as a 1911 guy), and I expect I'd have problems with it.
To avoid the bad DA trigger and the odd safety/decocker, some prefer the LEM trigger. You can convert a USP to almost any action you want, and I'd probably go down the LEM route if I were going to get a USP.
One prominent member on another forum, who spends a lot of time in Alaska has settled on the USP 45 for back country protection using .45 Super out of the USP 45, which he's had good success with. Prior to getting to the USP 45, he's been through a bunch of other guns including the G20, 1911, S&W 1006, and nothing else gave him the durability, accuracy, and reliability the USP 45 does.
I think if it fits you (it is a big gun), and you don't mind the cost of their pricey mags (hey, you chose an HK, and in .45ACP, so how much of a concern could cost be), and the odd proprietary light rail (I don't expect to ever put a light on a gun), I think it would be a great choice.