Hm, I'm using the Guest view add on now to achieve this. However, Guest view plugin "occupies" the custom template. I'm now stuck with blocks that already have a custom template for styling etc.

Still wonder why the setting in my screenshot doesn't work.
I've already ruled out caching problem.

Go to the MP and look for guest views and install that, you can't do it with advanced permissions

Sorry didn't read the whole post, what you have to do is make a new template, copy /packages/guest_views/blocks/content/templates to /blocks/templates and apply the custom templates to that file

Also, all users are also in the guest group that's why
It doesn't work

Thanks for your help!

Still weird that the Concrete5 permissions dialog allows me to make a block readable to guests only, which effectively doesn't work.

@Concrete5: Seems to me that "Guest view only" should be handled by the permissions section and not by custom php code.
If I have a block with 2 custom style templates, I will now have to build 2 extra containing the guest view code.
Basically I'm mixing logic and GUI now.

The problem hat u had was everyone was in th guest group, not just guests. (that's what mnkras was saying)

So is there any way of moving a user out of the Guest group?

(thanks every one for being so responsive :-)