I've been interested in CSS pre-processors for a while. They solve the inherent CSS problem of repeatable code blocks, and thus enable grid systems that don't litter markup with "styling" classes.
It seems to me that LESS and Sass always had one Achilles heel - debugging. Right clicking on an element that is styled incorrectly and inspecting it is absolutely essential to the process of styling a web page, and without that, these tools had no future. I now believe that this problem is behind us.
In addition to the existing solutions, Webkit now has built-in support for Sass debugging, and the option is now included in Chrome Developer Tools. Read more about it on bricss.net or benfrain.com.
As a side-note - there is a bit of a battle between LESS and Sass. I always preferred LESS because it seemed more cross-platform, and closer to actual CSS. However this development seems to suggest that tools are being developed faster for Sass. So it's now looking to me as though Sass will prevail.