I have had some of the best ski days of my life in Las Lenas since going the first time in 2001. It is by far the best advanced/expert terrain in the world for a ski resort, where you can ski right off the high alpine chair Marte and ski whatever you want, just need to sign a waiver that says you can't sue the resort if you hurt yourself out of bounds.
Due to its high elevation you are normally guaranteed good snow for your ski week. The only downfall to Las Lenas is that it is not the best organized resort and the chairs are very old and can be slow, and sometimes not open. When the upper mountain is not open the lower mountain can have long lines. To get up to the infamous Marte chair you need to take 2 slow doubles. When Las Lenas is on with good snow condtitions though and no lines, it is amazing. Accommodation prices have gone up quite a bit in 2008 so if looking for economic option there is staying 15 minutes away in Los Molles in cabins or 45 minutes away in Malargue.