The Arc'teryx Cerium SL is very well constructed and fits very well. It is intended as a midlayer, and it serves that purpose well. The lighter fabric does not hold up to wind and water very well, but it is so light and compressible that it is easy to throw into a pack or messenger bag. This is a great lightweight insulating piece to throw on under a jacket, and would be a little warmer and a LOT more compressible than an equivalent fleece. Arc'teryx uses a unique Down Composite Mapping to carefully place synthetic Coreloft insulation in key locations, but our reviewers found that kept the jacket wetter longer. Overall, it is a great jacket for a very limited range of purposes.
Arc'teryx informed us that they updated the colors available for the Cerium, as well as switching to a slightly longer fit.