I voted for 10-12
I'm a little surprised that the levels are so low; I have a feeling it may be because there are many more gamers who have not played higher levels than those who have.
Within the last six months, I played in a party where we got to about 15th level (highest ever for me), and I was surprised and overjoyed at the level of raw power we had as a party.
It did become problematic however, because the DM liked to railroad us a lot, and we were always able to outsmart him with our spells and items. It ended up where we had to just sort of humor him and 'play along' so that the story could progress. (In short, it felt a little like going through the MM alphabetically.)
As a party, we didn't care about the town we were trying to save anymore; we wanted to wander around the world and see what else we could dig up... The problem was, the DM had a pretty bad imagination, and our characters were
way more interesting than the world around them.
But in a good world with a good DM, (heck even in that crumby one) I would say the "sweet spot" is around level 12 or so. It allows for more player creativity.