Great advice by Strange Magic.
Having safety gear is a great set-up for making one less cautious in ones paddling.
Yet that applies to ALL safety items/issues. They are BACK-UPS. That includes any hardware you carry, and includes anyone you may be paddling with.
Prepare these back-ups carefully; paddle as if you had none.
Nevertheless, I still believe the single most-likely-to-save-your-life item you could carry in your PFD or kayak is a gps-equipped PLB. (The single most likely item to save you for most is appropriate cold-water-immersion clothing...but that's a separate matter). This is not theoretical...there are hundreds of lives which have been saved by them.
The chance of the unit itself failing to operate is exceptionally low; these have far higher construction quality that typical electronics. The more important qualifier is the time it may take for help to arrive. There was an illustrating case last year on Lake Superior in the fall (water temps likely 50 degrees). A couple wet exited their double and failed to self-rescue. They activated their PLB, and the rescue operation was promptly set in motion. Nevertheless, the closest rescue vehicle took three hours to arrive. If you're bobbing around in rough, cold water, that's quite a long time...likely far too long if you're not dressed for such an occasion. Still, they were rescued. Without the beacon, they almost surely would have perished.
So one definitely has to consider delays in the arrival of rescue due to remote location and/or prohibitive weather. Still, there's nothing else that is more reliable in summoning life-saving assistance.
If you study the course of actual survival scenarios, you will see one other clear benefit of having such a device: other than cold-water shock, the thing which is most likely to quickly do you in is panic. The panic of being in a life-threatening situation. Simply the fact of knowing help is on the way will keep many from crossing over into that debilitating panic.
So do yourself and your loved ones a favor: carry one. Just keep StrangeMagics advice in the forefront of your mind, and do not paddle in conditions in which you think you might need to use it.