For the people complaining about this jacket being too light, please read the name. This is an ultralight jacket and is built specifically to be light and packable so you can stuff it into your jersey pocket for emergencies like when the temps drop and you are 50 miles away from home.
And it is fantastic at doing that job. Packs up to the size of a tennis ball if you really squish it. No pockets. Just a zipper, some reflective stripes, and a grippy silicone strip on the rear hem.
Better than other ultralight jackets such as the ghost lite, especially at this price, because it does not look like a trash bag, is not see-through, and fits better.
The fit is perfect, and by perfect I mean that it fits like a thermal jersey. Not skin-tight, but snug with good freedom of movement and no bunching. This is also taking into account that you have to buy one size up for Castelli, as usual. It has stretch panels on the underarms and back too, so I can actually raise my arms above my head without the jacket riding up and with no gussets, which is impressive. You will probably be able to wear a thermal jersey under it, but not much else. I am 5'9", 170 lbs, 32" waist and have a larger upper body than the average cyclist.
Two considerations. First, buy a size up for a tight fit as mentioned. Second, Castelli does this weird thing with some of their clothes where they offset the two zippered halves at the collar by a quarter inch. I think they do this to eliminate zipper chafing without adding extra material, but it is definitely a design feature and not a defect. I don't like it, but i'll take it since the quality of everything else is outstanding.
This jacket is so nice that I might also buy the vest as well even though its already jacket weather.