dim4845508wrote a review of Mavic Echelon Short Sleeve Base Layer on August 28, 2013
Familiarity: I've used it once or twice and have initial impressions
The most important thing to know is the sizing. Mavic is a French company, however they list three sizes: Euro (Int'l), US, and Japanese; the size you see on this website is US. So the Mavic S and Castelli M are about the same size.
Now, to the rating. The material is nice but the stitching/assembly is not premium. For example, the panel that goes around the neck doesn't lie flat but has two creases. The little yellow 'M' on the front is attached with uneven stitches.
I've only worn it on a short ride (albeit hot) and it seems to wick fine.