What is your opinion on the ISC license?
Do you think it is suitable as MIT or Simplified BSD license replacement?
What are the pros and cons of ISC compared to MIT or BSD?
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"It's okay." Removing the now redundant language due to the effects of the Berne convention is good.
FSF has this to say:
... However, to help make sure this language cannot cause any trouble in the future, we encourage developers to choose a different license for their own works.
Which basically means that the terminology is just vague enough that people might shy away from using your software. The problem is that the license doesn't explicitly grant the right to distribute modified versions of the software.
If you decide to use this license, you'll need to expressly indicate your intent on whether users are allowed to distribute the modified version of the code. Even then, people will still shy away as that intent is not explicitly spelled out in the license.
Honestly, I'd recommend a different license. FSF recommends FreeBSD or Expat (aka MIT) over OpenBSD / ISC. This is the Wikipedia article comparing Open Source licenses.
In general, stick with the bigger, better known licenses. Why, you might ask? The less work people have to invest in order to understand your license then the more likely they are to consider using your software. If you use an obscure license, they'll simply move on to the next project or roll their own so they don't have to worry about potential legal issues.