Hi Oleg (the poster formerly known as Cornered {but cheery-I liked that} Rat);
I have two H&K USP .45 Compacts.
One is VERY accurate & precise. The other's precision is a disappointment, though quite good enough for "combat" purposes, that is, hitting the "A Zone" reliably at 7-8 yards.
The guns are not all identical, in other words, and I think one of mine is a bit of a "lemon", judging from others' experience and the other USP .45C I have.
That said, they're tough as bricks, and don't let anyone tell you different.
The grip is MUCH more comfortable than my Glock 30 (which I love, though it's a little like holding a 2x4), and the mags hold 8 rounds instead of 10.
Now, about your questions;
- owners of it, do you like it? Answer: Yes.
- how's recoil compared to compact .45 Glocks? Answer: The bore axis is slightly higher than the Glock, and compared to the Glock 30, I'd say the recoil is a bit more, but to a very minor degree. The "muzzle whip" accounts for most of this, due specifically to that higher bore axis, but it's really not much different in overall perceived recoil.
- how expensive are spare mags? Answer: About $45.
- what kind of holsters would work for deep concealment? Answer: It depends on "how deep" your concealment needs to be. I use OWB holsters (or shoulder rigs, in certain circumstances) with a covering garment (vest), and this gun disappears entirely with a Kydex belt holster from Blade-Tech or ANY of Lou Alessi's belt holsters.
- Does lack of ability to thumb-cock bug anyone? Answer: You CAN thumb-cock, especially if you get one with the standard hammer instead of the bobbed hammer, and as long as you have one of the Variants which permits the control lever to act as a decocker, there's no problem with safety if you decide to administratively lower the hammer. The standard hammer, with the longer spur, comes with Variants 9 & 10, on which the control lever acts as a safety but not a decocker, though the bobbed hammer can be exchanged for the standard hammer on any of the other Variants by an H&K armorer.
- anything else you'd like to tell me? Answer: 1.) I'd only carry this gun "cocked & locked", this being the only carry mode which makes sense for this gun. (That's why I own 'em, being left-handed, with Variants 2 & 10 permitting C&L carry without an ambidextrous safety being necessary. If I were right-handed, I probably would substitute a customized 1911A1-pattern gun for the USP, since the USP's DA trigger-pull is, well, not the best, though certainly workable. The SigSauer DA/SA transition is generally superior to that on the H&K USP series, IMNSHO, and IF I were going to be willing to adapt to the first-shot DA, all-subsequent shots SA transition (I'm not, I believe from experience in training that it's a liability), I'd choose the SigSauer system. I don't carry my SigSauer P220, though, for just this reason. If your intent is not to carry cocked & locked, by all means stick with the Glock 30, or the 36, if it ever becomes available.
2.) Get the flat magazine baseplates for CCW, rather than the extended baseplates. It makes a big difference for concealability, and your little finger will be fully supported by the grip frame without the extended baseplates, unless you have really big or fat hands, the grip frame of the USP .45 C being notably longer than the Glock 30.
- is it worth $750 (!), considering compact Glock (G??) is a bit less and holds more ammo? Answer: Who can say? In pure "value", the answer is probably "no", but these considerations are never based on simple economic comparisons. If you are able, and haven't done so already, by all means rent or borrow one and shoot it, carry & draw it from a belt holster, and definitely evaluate the DA trigger pull and DA/SA transition when actually firing the gun, not just by dry-firing, if your intent is to carry the gun in "first shot DA" mode.
Sorry for the nested parentheses in the above. Good luck to you!
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"Potius sero quam nunquam."