Had to decide between this and the 35L after deciding which FL I used the most on my 24-70L, and ended up with the 24L as it has better build quality, rubber gaskets, and ultimately a more attractive focal length for what I shoot, which is a good mix of people, landscapes, nature vistas, etc.
Focus speed/accuracy: Excellent, with one caveat - at apertures larger than f/2.8, hitting focus can be difficult with distances longer than 2 metres. Not an issue for me, though, and it can probably be corrected.
Build: As good as it gets. I´ve had it out in salty spray, no issues.
Great lens, will probably own this forever. Had it for six months or so.