I usually start my campaigns at about 1st-3rd level. 1st is good for some starts, not so much for others. I ran Freeport from 1st level and that went very much by the books, but for my current Snakemen campaign I wanted a 3rd level start as it gives the players a better choice of races (so that they can take advantage of ECL's should they wish, which 1 did). Plus its more fun as a DM I find, as you can throw some neater monsters at them right from the get go, especially as I have 7 players in my group so the party comes to about CR 5 right from the start.
I'm not sure what I'll start players at for my next campaign, though I am working on a GI Joe Spycraft campaign where the players would each have 3-4 characters starting at about 4th level. They would be given the mission brief and then select which character is the most suitable to carry out the mission objectives. Not sure whether I'll stick with that idea, or just have them with a single character each, the campaign is still very much in the plotting stage.