I know it's a guideline, not a rule, but I'm finding that I'm a lot happier with the flavor of the shots produced in the 40-45 second range. I can get an OK shot at 30 second extraction time sometimes, but 40 is much better, and generally I'd be more inclined to drink a 1 minute shot than most 30 second ones.
It's not just my machine either as I'm finding this on my Gaggia at home as well as the La Spaz S1 2 group at work. Lately at home I've taken to doing 20gram doses that extract to 40 grams give or take a couple in 40-50 seconds, and those are tasting really good, and though the doses on the La Spaz have to be much lower, and I haven't weighed the shot, I can say a similarly slow flow rate gives a much better flavored shot for me on the La Spaz as well. When I say better, I mean 30 second shots are at best just on the edge of sour, but not in a bad way, so I guess you could say they are balanced, but at that point the fuller flavor notes, sweetness, nuttiness etc. don't seem to have come out yet.
So what's the deal with that 30 second goal? I know it's meant to be flexible and adjusted to taste, but to me, based upon experimenting over the last several months, it seems more like the minimum, and not the ideal.
Then of course there is the argument of when to start the timer. Some people say start at the first drop, and some say start the timer when you hit the switch, but all seem to say the same 30 seconds. It seems to me a good flow rate starts with the first drops around the 7-9 second mark, so if I'm starting the timer at that point, I guess 30-35 seconds is pretty close, but again I find it confusing that everyone seems to say 30 seconds, even if they can't agree on when to start the timer.