I'm considering using the Twitter Bootstrap HTML/CSS templates which use Less (a sort of scripting language for CSS)
I've never used it so I was wondering if it's ready for "prime time".
I think the Twitter website has quite a few hits each day, so if it's good enough for them then I think it's ready for "prime time".
I'm not sure exactly where you think any problems might be, but LESS itself isn't supported by browsers. The LESS code you write is compiled into regular CSS, so the "support" is the same as CSS.
Furthermore, you don't even need to use LESS, you can use vanilla CSS instead.