My own taste is in strong contrast to Joshua's.  I think the semi-colon/break-if looks absolutely horrendous and ugly.  The 'while' clause looks obvious, beautiful, and intuitive.

However, I see the point made by a number of people that the 'while' clause has no straightforward translation into an unrolled loop, and is probably ruled out on that basis. 

That said, I think the version using Guido's suggestion (and already supported for generator comprehensions) looks fairly nice.  I.e. given a support function:

  def stopif(x):
      if x: raise StopIteration
      return True

We can express it as:

  primes100 = set(p for p in primes() if stopif(p >= 100))

Except I'm not sure I like the spelling of 'stopif()', since the repeating the 'if' in the function name reads oddly.  I guess I might like this spelling better:

  primes100 = set(p for p in primes() if still(p < 100))  # Obvious implementation of still()

The only change we need is the one Guido has declared as "do it" which is to make other comprehensions act the same as the instantiation with the generator.  I.e. this should work:

  primes100 = {p for p in primes() if still(p < 100)}

 

It doesn't *really* matter whether someone likes my spelling 'still()' or Guido's 'stopif()' better, since either one is trivially implementable by end users if they wish to do so.