It depends. What do you want it to do?
Every gun (every stinkin' one of them) is a compromise. You need to consider accuracy, reliability, cost, size, weight, caliber, capacity, ergonomics, materials, and a pile of other things that all add up to the pistol you see under glass at your local pusher's.
The Glock is a relatively inexpensive, extremely reliable, durable, easily maintainable pistol that will give you a lifetime of service. Parts are readily available. The pistol is everything you'd want a combat handgun to be - high capacity, easy to clean, easy to repair, difficult to break or make malfunction.
But that doesn't mean it's the best for everything. Glocks are accurate for an issued pistol, but there are more accurate pistols available. (OK, before you say it, I know there's at least one person out there who won an F-Class rifle match with a Glock 17. Your pistol is not normal) However, that accuracy comes at a cost. Either the pistol is more accurate but potentially less reliable, or it's significantly more expensive.
In action pistol competition circles (USPSA, IDPA), the Glock is one of the best choices you can make. In Production, you give up nothing to any other handgun. In Limited, you can make it run with the high-dollar S_I guns with minor modification. They're inexpensive, readily available, and easy to run.
Also, Glocks truly are a platform. Get started with a 17, and the world opens up. Change size, change caliber, it's the same basic gun. I don't think any other gun company offers as many different options in size and caliber with the same action as Glock.
So why don't I own one? I had a Gen 3 23. I really, really, really wanted to love the gun. But the truth was, I could never get comfortable enough with it to be accurate. I owned it for about 6 months, and put several thousand rounds through it. I just never got to the point where I could be effective with it.
Also, Glocks can be somewhat problematic for reloaders. I think 9mm is probably fine, but I'm still personally leery of loading .40 for a Glock. Also, the polygonal rifling makes lead bullets a no-no (yes, I know there are folks out there who shoot lead through their Glocks, there are also those who've done so with less positive outcomes).
All in all, the Glock is a fantastic gun at a reasonable price. There are others in that price range that offer different design choices that are probably just as good, but maybe not as well adoped.
If you like them and can shoot them, there's no reason not to own one or a bunch of Glocks.