It uses pretty much exactly the same syntax as CSS itself; it only adds stuff and doesn't change stuff, meaning that existing CSS can be used. This is not a CoffeeScript / HAML equivalent for CSS, this is actually nicer than the language itself.
See http://lesscss.org/ for a tonne of good examples. My favourite features are variables, mixins and nesting, but I'm not a designer - they might find the colour functions very useful (a stylesheet written using LESS CSS with all the colours without each other would certainly be easy to colorize!).
There are a couple LESS CSS compilers for PHP, but I haven't really looked into them yet.
- Adds features to CSS and makes writing CSS very easy - this is a HUGE advantage
- Can handle minification
- Needs to be compiled (but can be cached)
- May cause some confusion for style authors (but they don't have to use it, and it is very easy to pick up anyway).