That's a bit of a loaded question and as others have touched upon, it depends on what the definition of "mystical experience" is.
I've read many people describe "mystical experiences" that, to me at least, have no mystical aspect whatsoever.
Then there are others that mimic a "mystical experience" that weren't in any way "mystical." They can range from drug experiences to just momentary experiences that seem somewhat "mystical."
Then there are the "internal" experiences that, even though they might be "mystical" in the sense they actually happened to us (just as we define "happened" on our everyday level of consciousness) and weren't just "imagined" the way we define "imagination," when we go beyond our so-called normal senses, much of what we experience on "higher planes" is "maya" (or "illusion") too.
At the lower end of the journey inward it's simple to describe as it is mostly lights and colors and sounds that are familiar to us.
Once you move higher into the "astral plane" it becomes more difficult (and things that are usually simple, such as sounds, begin to attain other qualities) and when you reach the "causal plane" it becomes very difficult for many reasons, but perhaps the biggest is that you are seeing things and experiencing things you had no idea even existed, and so have no way to describe it because it's not even made up of "parts" you are familiar with.
Once you move beyond that, it's just indescribable and is an "area" only the "highest" attaining mystics have ever experienced, but they have all said they are unable to describe most of it.
All told, very few would even attempt to describe any of this as only the highest attaining mystics ever reach a state where they have completely lost the ego and have merged with God.
Those who have gone that high, can describe whatever they wish. But the ones who have done so have absolutely no wish to talk about it as they are fully enveloped in intense bliss that goes way beyond anything you can imagine. That is all that matters to them and since they have no ego whatsoever they have absolutely no desire to even talk about it.
While many have reached that stage, the only ones you've ever heard from are the famous mystics throughout history who came here specifically to pass on their experiences and to teach to those who were ready to hear. And many of the people we've all heard of as being "mystics" were only mystics from levels lower than the "ultimate" highest indescribable abode (which was attained by mystics such as Kabir and Nanak, etc.). The lower levels, while still a remarkable achievement, will only result in giving one a long span after death in one of the "heavens" but will end with the mystic having to come back into the cycle of births and deaths and start all over again.
For anyone who still retains any semblance of ego, attempting to share any inner experiences at all, will result in them losing whatever they had attained - as has been proven time and time again by many of the "lowest" attaining mystics such as Osho, Adi Da and numerous others who manage "some" mystical accomplishment, and gain certain "powers" of various sorts, but then they use those powers toward ego fulfillment and over time they lose it all. You can find countless stories of "gurus" who had a bit of internal accomplishment and attained great magnetism and heightened intellectual abilities, etc., and then used the little they'd gained to make themselves look great, and usually toward sexual conquests, (there are very few gurus out there who die without being attached to numerous sex scandals as one has to accomplish a lot internally before "lust" completely leaves forever, and meanwhile it serves as a great "test" to determine whether one will ever attain greater stages internally) and they always come violently crashing down.
And all those gurus/teachers were only at the very beginning stages with far to go, but mystics are always granted much and tempted with incredible things, in order to differentiate those who are all about ego from those who are truly involved in their mystical pursuits solely because they want to "go back" to God. Those who are truly about "God" and not about "I" are granted powers and abilities that make what the Oshos & Adi Das of the world ever attained look like a joke!
But, as they are usually forewarned, if you use it, you'll lose it. And MANY have found the hard way that is 100% fact.
So, if you have anyone relating their "mystical experiences" you can almost guarantee they either weren't mystical at all, and if they were mystical, they were VERY much "minor" experiences. Because you can't get much until you have gone far and you will never go far until you have proven you will keep your experiences to yourself.
And most importantly of all, a "student" can NEVER go beyond the state attained by his/her teacher and the highest anyone can ever attain without even having a teacher is no more than attaining the very beginning of "path." (And without a teacher who has already been there you will have to do 10 times the work to even get there.)
So, if someone is trying to tell you about their "mystical experiences" the odds are GREAT that they aren't mystical at all (if they truly even experienced anything), and if they were of a mystical nature, then they were likely not much at all.
If you want to read some true mystical experiences do some digging. The great thing is that the "Truth" never changes and one True Mystic has the same experiences as another True Mystic does. That's one of the easiest ways to tell who's who throughout history.
Best of all, the Great Mystics have put it all out there so you can verify it all yourself. You don't have to "wait until you die" in order to see whether your beliefs are true. When you are able to "go inside" you experience death. That's not an "expression." It's fact. You see and experience everything. The heavens and all that's associated with it. You "die while living." As the Christian St. Paul said, "I die daily."
A GREAT book online that describes it all is called "The Path Of The Masters" by Dr. Julian Johnson. He was an amazing man who accomplished much in life, beyond "merely" being an accomplished surgeon. But as he got older he found himself being pulled in a search for the ultimate "Truth" and he ended up "finding" a guru known as "The Great Master." (His name was Sawan Singh.) He ended up studying under him and writing some books about it, and "The Path Of The Masters" was the greatest of them all. It really lays it ALL out there and is somewhat mind-blowing at first, but anyone who decides to test what Dr. Johnson says, will find in time that it is all factual. (I've met several people over the years who proved that to themselves.)
But, even if you have no interest in "trying" it yourself, it's still a fascinating read. I've seen a few different people have posted the book on Scribd where it can be downloaded for free and someone else has a copy online as a bunch of Web pages, though I don't know the address. If anyone here has a true interest in mysticism at its highest possible form, take a look at that book first and you'll at least get a sorta "road map" of where the journey will take you. The book is around 60 or 70 years old but it's still held in awe by mystics all over the world and I've read it 3 times over the years (and constantly seeing things I previously failed to pick up on.)
It's a great starting point and if you find your interest grows, there are lots of other sources of material out there. If not, at least you got entertained and learned a lot about the highest possible mystical path.
_