Building a small deck and put in the posts yesterday, decided to try this stuff I've seen at Totem and Timbertown:
http://www.chemque.com/pages.php?page=Fast_2K
Seem to work awesome! Saves a ton of time and even more effort; didn't have to carry a whole bunch of bags of concrete down, mix it, etc.
Very easy to work with, gets firm in a few minutes. Had six holes, each about 3-4' deep, 9" diameter, and with the 4x4 post I used 8 bags, 1 bag each and then 1 bag to top up 3 holes.
Only downside is cost, they're $17 each or something like that. Probably makes money if you're a professional since you can do more in less time, more expensive for a DYI though.
The guy I had dig the holes said he's heard of only one bad story; one guy doing a fence put down the cardboard tubes first but didn't backfill around the tubes very well, and of course the foam stuff doesn't weigh hardly anything, so the fence blew over in a wind, pulled the sonotubes and everything right out.
Anyway just wanted to share a product that was new to me that I really liked, saved a ton of time and effort.