So I've tryed to make somethingl ittle demoeffects, but it's kinda hard so is jME good for making demos? I tryed OpenGL, but I am so new to 3d that I dont understund much about it and I dont get JOGL work well on my other computer. So I heard abou jME and it looks much easier so is it good for demos? 
Of course it is 
- if you're not talking about 64K-demos or similar, that is
You should define what "demos" are for you, if you would like to get more precise answers. If you're "new to 3d" it's probably better to start learning 3d and GL concepts before expecting to build really cool demos ;). And if you were not able to get JOGL running with your own apps you probably better start with easier Java stuff and find a nice IDE, to learn one thing after the other.
I get JOGL to work on my second computer, but in my other computer I didn't get it to work (I don't get it to work in anything. I use ubunt, but something is wrong in that computer. I must restart it about 3 times before it starts to work!). And I tryed JOGL and its kinda hard, but I will test jME! 
Go for the Java Webstart demos… if they work, then you should be in good shape.
Depends on how quickly thrown together of a demo you need. You might also want to take a look into the Blender Game Engine. It's good for very quick demos, in my opinion.
The only problem is the documentation. I had issues finding good docs on the Python controls. It's being updated, so…I'm not sure of the current status. It's tied pretty closely to the models and Blender attributes and such - so it's easy to set properties, including physics. The movement and changing of states and such can all be controlled by adjusting values in dialog boxes.
Make no mistake, jME is very awesome, but if you're looking for something simple and quick, you may want to give it a go. Or you'll get lost if you can't find good docs. Either way XD