So… depends on the jacket.
I’ve have a Mavic HC H20 waterproof, and it fits me like a glove, like a glove. I couldn’t really wear it to walk in, as it fits me so close it only works on the bike, which is why I love it.
I’m a medium in their sizing, usually a small medium in most brands. I’m 11 stone, 6 foot and athletic in shape.
The cut is short and technical, so if you want something that fits you like a normal jacket, then look elsewhere.
But… if you’re after something that fits like a Assos or Castelli road jersey should then they’re ace.
For me, they’re made for athletic builds – and if you’re not, I think it just wouldn’t fit or look right.
On another note, I’m a medium in Rab gear – but could go small, and Haglofs medium too.