The USP was designed to shoot .40, rather than being a 9mm that was 'scaled up' like most guns. It is one of the better gun platforms to use with .40--the dual recoil springs work very well. Note--the Compact models don't (or didn't) have dual recoil springs when I shot one.
Yes, it has a high bore axis. You won't notice it like you would with an XD, for instance, due to that recoil system.
The Tactical DOES have a nicer trigger than the stock USP. I'd have to agree--out of the box a stock USP DA trigger pull is pretty heinous. They do shoot in nicely, though. After 1000 rounds, it's a whole different ball game, and it just keeps getting better. Before you know it, it goes from 'heinous' to 'marginal'. <g> Again--the Tactical doesn't suffer as much from that.
The Tactical also has the O Ring seal on the barrel, and should prove to be one of the more accurate handguns you've shot. With an outstanding recoil system, designed for the round you're shooting, it will be pleasant to fire. HK has the fastest mag change in the business (use your trigger finger, not your thumb to drop the spent mag).
Yes, if I had the discretionary cash laying around, I'd pick it up if the price was right. Used, it'd need to be $750 or so.