[gress]

Let is fine for defining constants, but it should also be used whenever possible for two reasons:

1. It gives you additional safety against accidental reassignment.

2. It gives the compiler a major optimization hint since the value is not a variable.

[Jweb_Guru]

The latter isn't really true when it comes to local variables. Compilers are pretty good at figuring that sort of thing out these days.

[gress]

It is one of the reasons stated by Apple at today's WWDC session.