They tend to look very structured and could be considered cold. Having said that, modern fonts can look really eye-catching and very elegant at large sizes. They are not suitable for large amounts of body text, either on the web or in print. When used for body copy in print, an effect called “dazzling” occurs, the thick lines become very prominent while the thin lines almost disappear. It’s best to keep them for headings and sub-headings. You may not use the modern typeface too often, but as a designer it’s nice to be able to pick out and recognize font categories.
You probably have a number of Modern fonts on your computer already. Some examples are Didot, Onyx and Times Bold. I’ve picked out seven more which are available to download for free.