Having put abut 700 rds through my XFive now, here are my findings.
General: I have read numerous complaints about its tightness. To me that is a plus, not a negative. After all, one of the promises was its accuracy. Yes, it is very accurate. I have shot only hand loads through it. Some loads would not reliably run in the gun, due to their lack of pressure to fully extract and eject rounds. As the magazine capacity was diminished it got better. A very, very firm grip helped too. After increasing powder charge a tad, it became 100% reliable, and the increased loads were still under 130PF. With lighter loads, fewer rounds in the mags help, as does a very firm grip.
Battery: I never encountered a failure to go into battery during normal firing of the gun, excepting the failure to extract and eject as mentioned above with low PF loads, which obviously prevent the battery condition. If you attempt to ease the slide forward, it likely will not go into battery. I have installed a GG guide rod so that I can control which recoil spring to use. The GG guide rod came with a 15 lb. I am running a 14 lb, which works fine with my hand loads. If I bump the loads up a tad, then I will experiment with the 15 lb spring, or even run the factory captured spring/rod assembly. During normal firing, I am sure it will strip a round off the follower, chamber and go completely into battery with an even lighter recoil spring.
Accuracy: With my hand loads, my 65 year old eyes and the .90 front sight it is a challenge to really get the sight picture, I like. Having said that, I have fired many groups under 2.5" at 50 yds with this gun, simply shooting prone. I think the gun will shoot 1.5 - 1.75" at 50 yds with the right ammo, the right eyes and the right talent behind the trigger.
Hand loads used: I have tried Zero 115 JHP old stye, Zero 125 JHP Conical, Hornady 125 HAP, Bayou 124 TCG, Bayou 120 TCG. The same mild powder charge of WST was used across the board. No changes to powder charge and no changes to OAL die setting, but there was slight variation in OAL due to slightly different bullet noses. They were pretty much 1.075 +/- .005. I knew all the loads would be soft (120 +/-PF) so I wasn't concerned. All the jacketed loads were too soft to run the gun with more than 2 or 3 rounds in a mag. After, I kicked the load up by 2/10ths it ran flawless. (Except for the 115 gr bullets. They still needed more powder to make them 100% with topped off mags.) Magazines locked back etc. After the increased powder charge, my 125's JHP were running about 1000fps and the 120, 124 BB were running 1050 - 1020 fps respectively. I did not chrono the 115 JHP. All very accurate, all had more potential with some tuning for the specific bullet/weight/material. Maybe a different powder too. I had, and like WST for .38 Spl and .38 Super. That is why it was used.
Trigger: I am a revolver guy, so I found the trigger quite acceptable, but I have replaced it with Gray Guns Straight Face PELT. Before it was in the low to mid 7 lb range with Lyman Electronic tester, depending on where the rod was placed on the trigger. After the GG PELT trigger it dropped considerably. Way lower than expected or advertised. I do not know if this was because I "over wrestled" the spring into place or because of the way which I lubricate, or was I just lucky. (I use a dab of STP on contact surfaces of trigger, and on slide rails during warm weather.) After the PELT, the same trigger weight test results ranged from right at 4 to about 4.5 lbs. I had some break below 4 lbs. For me, if a revolver or DA, or a striker, I just "keep the trigger moving" and it works well for me. But again, I am a revolver guy so a 6-8 lb trigger is what I am used to, which has to be moved almost half an inch. So the Sig 320 trigger was fine to my feel. I commit to the trigger and keep it moving until it breaks.
Sights: I am a patridge front sight guy. The fiber optic is coming off, and a wider patridge is going on. Solid click adjustments on rear. Good sights IMO.
Grip: Too slick for my taste. Added skateboard tape. I have seen the pre-cut tape kits, but I need tape added to the smooth area of the back of the grip. The pre-cuts only cover the existing textured areas.
Magazines: I have not used the 21 rdrs. I bought 4 17 rdrs and had to modify the the base pads so that they would clear the magwell. A bit of a pain, but after I got the first one done, which gave me a template to how much, and where material had to come off, it took less time to do 3 than it did the first one.
Holster: Red Hil Tactical ordered, not yet received
Closing: As I said, I am a revolver guy. If I could only have one handgun, it would be a 4" S&W .357 magnum revolver. Either a Model 586, 686 or 19 or 66. However, if one of those were not available, and I could only have one, then the Sig 320 XFive is rapidly winning me over. It is that good, so far, IMO.