I'm 5'8" 145 and the small is definitely the one to get. It fits properly on it's own and has just enough room for an Atom LT or lighter weight fleece underneath (100 wt fits well, 200 starts to get a little snug). Anything larger and I'd likely be swimming in it.
PROs: Versatile for moderate to sunny conditions and changing activities/sun direction.
Looks good - like a baseball cap when the flap is stowed.
The oversized brim doesn't look oversized. It's super packable.
The brim is floppy and can get deformed, but doesn't stay deformed. Just lay it out and it'll go back to it's natural floppiness. I've taken care to avoid creasing the brim, so I'm not sure if it would rebound from a hard crease.
CON: Sizing for medium heads. When the flap is stowed, the size effectively gets smaller. I'm a size medium head (~22.5 in circumf or 7 1/4 diameter) and always fit a size medium helmet in any brand. The size "S/M" is just too snug to be comfortable for long periods when the flap is rolled up and buttoned. I want to try it in the L/XL size, but I'm concerned it might be too loose when the flap is down!