> There is also another advantage to Less/SCSS, which is that existing CSS files are already compatible
Not sure about SCSS, but in my case Less choked on some complex hacks like IE filters. Not arguing about their evilness, but the fact remains—not always you can simply rename .css to .less and gradually start using new features. (In the end I kept CSS as is, with a separate Stylus file with overrides and new styles. SCSS wasn't an option, and I have a feeling it also suffers from these issues.)
Not sure about SCSS, but in my case Less choked on some complex hacks like IE filters. Not arguing about their evilness, but the fact remains—not always you can simply rename .css to .less and gradually start using new features. (In the end I kept CSS as is, with a separate Stylus file with overrides and new styles. SCSS wasn't an option, and I have a feeling it also suffers from these issues.)