I've never used the LESS framework when it comes to big CSS projects. My choice is entirely based on the primal design of CSS which hinges on simplicity. Adding new features to the CSS language is something that only the W3C CSS Working Group can handle without breaking the original design. CSS is, together with JavaScript, one of the most popular languages on the web. Its success is entirely due to its simplicity, to the fact that also non-developers can use it or at least modify a style sheet without being programmers. On the contrary, LESS introduces concepts that are widely used among programmers, such as variables. Many supporters of this framework claim that LESS improves productivity. But we're actually talking about layout styles and design, not about something that seems to come directly from an automated factory. CSS is design, not programming.
As a JavaScript developer, I can say that when I write my code I'm actually programming. Instead, when I write my CSS, I can say that my work is exactly the same as the daily work of a designer. Designers use fonts, colors, boxes, background images and page effects to create something very similar to a little work of art.
When I write CSS code, my mind is free of all those programming concepts that are a daily part of my work as a programmer. Because CSS is not a programming language, but a style language.
In the early 90's there was an heated debate among developers about the style language that browsers had to support in order to display web documents other than the famous Mosaic browser.
DSSL, DSSL Lite and CSS were among the favorite candidates to become the official style language for the web. DSSL was actually a complete programming language, while CSS was a very simple style sheet language.
History says that CSS won the race and DSSL fell into oblivion. DSSL was too complex to be adopted by web developers. CSS is the result of a battle between complexity and simplicity. Web developers and the W3C itself chose simplicity.
I'm not a big fan of CSS3 animations either. Keyframes actually work as variables. This is not the original design of CSS. CSS is designed to be simple and accessible to everyone. Any other use will result in a costly mistake.