When I was looking for a new competition handgun, in 45, I looked at the Mark 23, the Tactical, and the Expert. The Expert had just enough over the Tactical, in barrel length, OAL etc. to make me choose the Expert between the two. I REALLY liked what the 23 had to offer, but I just had NO use for the threaded barrel. So I ended up with the Expert. Which was about the closest to the 23. (at that time)
After using it in competition for a few years, I can say this, it IS an extremely accurate and reliable handgun. Never had a FTF or a FTE. MANY
figure 8's (and one hole double taps) in the targets, shooting at competition speed.
Couple of points if you are considering an H&K. The grips are LARGE. The handgun itself is quite large. Especially the 23, with an OAL of 9.65 inches, compared to a regular USP at 7.87 inches. Some people are not comfortable with this large of a handgun. Another point is the mag release on them. Personally, I LOVE it. MUCH faster, and came completely natural to me. Some people that are used to the "conventional" placement, can not get used to the new location. Lastly, with the polygonal barrel, you are not supposed to shoot lead bullets through it, just FMJ.
The 23's are available to citizens, and used to run around $2.600, but I have seen them lately around $1,700. A little less if you are patient. If you decide to get one, after trying one (or at least holding one to get the feel of the size) I do not think you would be disappointed with it's performance. Question is, do you need a threaded barrel, and are you comfortable with this size handgun?
I liked the Expert so much, that I got a regular full size USP, because the size is a bit easier to carry concealed. I used it in a few IDPA matches. While the accuracy is not quite as good, it has been just as reliable as the Expert, and I have NO other complaints about it. If you plan to carry it, I would recommend the regular full size USP. For some, it is a bit large to conceal, much less the 23, the Tactical, or Expert. If you are going for a suppressor........well........