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###What's wrong with !important:

###When is it okay to use:

###Conclusion

What's wrong with !important:

When is it okay to use:

Conclusion

Yes, !important is generally bad practice.

###Why it's bad practice:

Using the !important declaration is considered bad practice in most cases, as it most likely indicates poor CSS architecture. There are cases in which it's tolerable or even preferred, but make sure you double check that one of those cases actually applies to your situation before using it.

Yes, I'd say your example of using !important is bad practice, and it's very likely it would cause undesired effects further down the line. That doesn't mean it's never okay to use though.

###What's wrong with !important:

Using the !important declaration is often considered bad practice because it has side effects that mess with one of CSS's core mechanisms: specificity. In many cases, using it could indicate poor CSS architecture. 

There are cases in which it's tolerable or even preferred, but make sure you double check that one of those cases actually applies to your situation before using it.

button { 
    color: black; 
}
button.highlight { 
    color: blue; 
    font-size: 1.5em;
}
button#buyNow { 
    color: green; 
    font-size: 2em;
}

button.highlight {
    color: blue !important;
    font-size: 1.5em;
}

button { 
    color: black; 
}
button.highlight { 
    color: blue; 
    font-size: 1.5em;
}
button#buyNow { 
    color: green; 
    font-size: 2em;
}

button.highlight {
    color: blue !important;
    font-size: 1.5em;
}