Mmm, yes, Glocks. I'll do a write up on my Glock 22 that I've been having modified, but for now I'll give my overview of the Glock lineup.
In full disclosure, I don't like Glock. They don't fit my hand all that well, the grip angle is not what I prefer, and the trigger isn't to my liking. I *can* shoot a Glock reasonably well, I'm just not as fast or as accurate as I am with my favorites. Its not a matter of familiarity, I've got thousands of rounds through Glocks thanks to it being my issued weapon with the PD. I am currently working on a little project I'm calling "Make my Glock shootable" in an attempt to overcome my objections, though. I've got a Glock 22 that I bought back from the PD when we went to the Gen 4 Glocks and I've had a trigger job done and installed a grip force adapter that changes the grip angle. Sights are up next.
Glocks are marketed as being extremely reliable. This is true. However the gap between Glock and every other modern quality pistol is pretty low in real use. I've got about tens of thousands of rounds through a hand full of handguns combined (CZ, Springfield, Sig) with never a malfunction with factory ammunition. However Glocks are very tolerant of misuse and neglect. I'm not the kind of person who never lubes his pistol or drops it in a mud puddle and then just shakes it out, but they exist, and Glocks will continue to run longer under those conditions than many others.
Glocks are "combat accurate". They aren't target guns. Because of their looser tolerances, they just won't hold as good a group as a tighter gun. For me, I can keep all my shots with a full sized Glock in a circle the size of a basketball at 25 yards. With my GP100 I can hold them in a circle the size of a baseball. Functional difference in a self defense shooting? Not much, either circle is still all in a human sized target's chest. Note that the Glock is capable of better groups, as I said it doesn't fit me that well, but even the best Glock shooter is never going to hold the same group as a similarly skilled 1911 shooter, target revolver shooter, etc. The slightly sloppy accuracy is a tradeoff resulting from looser tolerances in the name of reliability when full of goo or improperly lubricated or using lousy ammo, etc. etc.
Glocks are a true family of weapons. Magazines interchange from the bigger framed pistols in a given caliber to the smaller ones. If your primary pistol is a Glock 22, you can use a Glock 27 as your back up, and all of your G22 magazines will also feed your G27. If your primary is out of the fight, you aren't also facing reduced ammo capacity. The trigger and grip feel is same-same across the lineup (obviously small frames will have shorter grips and won't be the EXACT same, but if you like one Glock, you'll like them all).
Glocks are easy to take down and clean, the finish is tough, the polymer frame can't rust, and they truly will take a beating.
Glocks are relatively inexpensive, especially for the quality that you get.
Glocks have no thumb safety and are simple to learn to use properly.
Now, the downside of Glocks:
With the factory barrel, you can't shoot lead reloads. The type of rifling they use will fill with lead, causing the Glock to explode. Seriously. This is what started the "Glock grenade" jokes, early on people didn't realize this. The Glock does not have a fully supported chamber, it has polygonal rifling, and both of these things are bad for reloaders. If you don't care if your brass has a little bulge in it and you never shoot lead bullets (most factory ammo is copper jacketed) then you don't need to worry about this.
The trigger isn't that good. This is something of a gun snob comment, but it *does* impact your ability to shoot well. The Glock trigger creeps, its gritty, it stacks, and it has quite a bit of over travel. The reset is on the long side. Gen 3 and 4 are a bit smoother than the earlier Glocks (at least Gen 2, I've never shot an original Gen 1 Glock), but still aren't what I would consider a good trigger. The ONLY major brand polymer pistol with a worse factory trigger, IMO, is the Smith & Wesson M&P lineup (excluding the Shield, which uses a different trigger system than the full sized M&Ps). This can be overcome with aftermarket trigger components and a trigger job by a competent gun smith. Note I said competent. Again, due to looseness of tolerances, Glocks need a bit of massaging. The guy who did my trigger tried 5 trigger groups before he found the one my Glock liked best, then polished everything and hand fitted it.
The grip is pretty fat. The newest Glocks (Gen 4) have backstraps that come off and can be replaced, but you can't really make it much thinner. If you have smaller hands or just don't like the fat grip feel, there's nothing you can do about it. Unless you are willing to grind the polymer down, you can't change the finger grooves. Glocks are not very customizable for the individual, but this is true of most polymer pistols.
You have to pull the trigger to break the gun down. The slide won't come off otherwise. You SHOULD ALWAYS MAKE SURE ANY FIREARM IS EMPTY BEFORE CLEANING, but the Glock is very unforgiving of breaking this rule. Its the only "not newbie friendly" aspect of the firearm.
I also don't like the grip angle. Glock fanbois will go on and on about how its not that different and a few degrees of angle don't make any difference. It might not TO THEM, however the Glock does not natually point for me. I have to bring my wrists further down than where they naturally want to go when I point or I will shoot high. Not enough to miss at 10-20 yards, but the group does start working its way up the target. Why does this matter? Speed. A gun that you naturally point is faster to aim and faster to get back on target. Natural pointing varies greatly from individual to individual, so this may or may not apply to you.
Summary:
Glocks really are good guns. They do everything well, they are newbie friendly, they are deep into "good enough" territory for experienced shooters in deadly situations. They are cost effective, they are training effective (as practicing with one translates well into practicing with any of them), they are maintenance effective (little down time in stripping and cleaning).
*I* just don't care for them because they aren't the best fit for me. I'm to the point in my shooting skill levels that I'm looking to shave tenths of a second off my times. I *need* a gun that works naturally with me. Would I trust a Glock to protect my life with? Absolutely. I do every time I'm back in uniform for a detail, as I have to carry the issued Glock in those circumstances. Its always in my trunk in my callout bag, along with a full duty belt, uniform, boots, etc. Would I prefer it? No. For the simple reason I'm tenths of a second faster and inches more accurate with my Sig, and in a gun fight I want every advantage I can get.