No-one has ever confirmed it, but I believe I have experienced a jhana. They're kind-of unmistakable, being like nothing one has ever experienced before. It was the "jhana light" kind.

I would say this: it is the most lucid state I have ever been in my life.

I do think, as well, that you cross some kind of line when you experience a jhana. You have conclusive evidence that there are states of mind that do not depend on the sensuous world.

You realise that the Buddha could not have just been making stuff up; he knew what he was talking about. Care must be taken though, as jhana is not nirvana, so it's an not irrefutable understanding of the whole of the Buddha's teaching.

I think it's slightly the wrong way of looking at it to say if the ability to concentrate and understand difficult concepts is increased. Jhanas are about letting go, so you conceptualise less, not more. However, the overall feeling of (mental) wellbeing shoots through the roof at the time of being in the jhana. Jhanas are blissful states. You will enjoy them.

I can't say I've noticed radical changes in my daily life. Perhaps it's a more subtle and ongoing thing, though, rather than being "in your face". When you come out of jhana, normal daily life still goes on. You still get angry, happy, and stuff like that. Perhaps there's a change of perspective. Experiencing a jhana once is just a stage along a path. It's not the destination. Plenty of backsliding is possible. I guess at least you know that you are on a path. Most people spend their whole lives unaware that there is a path.

Some interesting things can happen. If you have good equanimity, for example, you can experience pain, it will be painful, yet your mind won't be agitated by it. You get an understanding of what it is to be "free" from something.

The jhanic states aren't permanent, of course, so you do go back to feeling pain and stuff the way that you "normally" do.

Just my own perspective. I hope that helps a little.