The only experience I have with Gen guns is with some G22s, but I'll throw in my $0.02.
Many years ago, I bought and carried a used 2nd Gen G23 for my PD job in an attempt to gain ammo commonality with most of the rest of my coworkers, who had bought a mix of 3rd Gen G22s and G23s. I reload my pistol ammo and found that my G23 had such poor case head support at the feed ramp that I literally could not work up a decent practice load without getting bulges in the case at the feed ramp. Even factory ammo bulged the case at the feed ramp when fired. Strike #1. The first time I qualified with it, I had a catastrophic case head separation which started at the massively unsupported feed ramp. Case head let go, magazine and extractor blew out and I thought I had lost my trigger finger. Luckily, my finger was OK but, at the time, it hurt bad enough I thought I was really hurt. Strike #2. I also found that the G23 (and later, when I shot a couple, the G22s also) displayed quite a bit more muzzle flip during recoil than any other 9mm or .45 gun I have ever shot. Noticeably slowed my time between shots. It wasn't as bad as some make it out, but a lot more than it should have been and annoying as all get out. Strike #3 and it was out. I went back to my G21 for my duty gun.
Fast forward to a few years ago. My PD got a new Chief who decreed that everyone would carry G22s for duty guns. He bought 8 Gen4 G22s to cover those that didn't already have a G22, with intentions of buying more the next year as the budget allowed, until everyone was carrying an issued Gen4 G22. He ended up getting another job about 6 months later and our policy went back to allowing personally owned duty guns if you wanted.
As the head firearms instructor, I was the one that inspected and test fired all the new G22s, as well as being the first one to get one issued. Let me tell you, if I hadn't already developed a hatred of the .40 round, the Gen4 G22 may have changed my mind and made me a believer. The dual recoil spring assembly tamed the annoying muzzle flip and I'd estimate that recoil and muzzle flip was roughly the same as the 9mm guns with a single recoil spring assembly. Case head support improved to the point that I'd call it fully supported. MUCH better than the previous generation guns and I noticed no more bulging brass on fired cases. Case head support was good enough that I wouldn't feel bad about reloading for .40 any more, as long as it was for a Gen4 gun. I really like the grip texture. Aggressive enough to give a good hold, even with sweaty hands, but not so aggressive that it wore out shirts. I never really carried the G22 concealed, so I don't know how it worked against bare skin, though. The bigger magazine release was also very nice and worked well. I didn't use any of the larger back straps and loved the smaller grip diameter. It made the gun fit my hand even better than the earlier generations guns, which I thought fit me well before. I think the geometry change with the connector in the trigger mechanism made the trigger crisper. A little heavier but had a much more crisp feel.
Perhaps the biggest change I noticed was the accuracy. I honestly don't know what Glock did, but all of the Gen4 G22s we had were laser accurate. One of the most accurate auto pistols I have ever shot. It seemed like all I had to do was point the gun in the general direction of the target and I would hit it. I was VERY impressed with the accuracy improvements.
I have never had a chance to shoot any other Gen4 guns. I'd like to try out a Gen4 G17 or G19 and see how they work. I know when they were first introduced, I had read about trouble with the dual recoil spring being too heavy for the 9mm guns and not working well. I also know that Glock introduced new RSAs for the 9mm guns. If they shoot as well as the .40 guns, I could see a Gen4 G17 or G19 replacing my current 3rd Gen G19 as my duty gun. I like everything else about the Gen4 guns and think I'd like one, if the troubles with the 9mm Gen4 guns are worked out. I'll get a chance to try one out sometime.
Bub