[maccard]

Like everything in C++, it has it's share of footguns

    if (Foo* f = GetPtr(); f->HasValue()) {} // wrong; f can be null

vs

    if (Foo* f = GetPtr(); f && f->HasValue()){}

Is probably the biggest pitfall. Especially if you're used to this:

    if (Foo* f = GetPtr())
    {
        f->DoTheThing(); // this is perfectly safe.
    }