I'd heard about the PAC-12 a few years ago, but I wasn't familiar with what it actually was. With the Internet at my fingertips, I found it in no time, and began to read about the details. It sounded good; certainly worth a try, so I made one. I always take this antenna with me because I may wind up operating in an area where the trees (if any) are in the wrong place. This happened at the unlikely QTH at Turkey Run State Park in Indiana. There were millions of trees around, but the ones at our campsite were just not in the right place. So I wound up using the PAC-12, and it worked great. It has the same thread at the base as a camera tripod, and I have a small (6") tripod that is dedicated to the PAC-12. If the ground is rocky, or if you're operating from a parking lot, just set up the tripod, and you're in business. I've used three and four radials, but I think twelve radials that are ten feet long would be optimum. There's been a lot of discussion about the number of radials and how long they should be, but the consensus seems to be that 12 is an optimum number, and they should be as long as your antenna is tall (see August 2003 QST, "Optimum Radial Ground Systems", by Robert Sommer, N4UU). I've only used the PAC-12 on 20m because 15m and 10m are not very active right now, and I don't think the antenna would work as well on 40m as it does on 20m.
The construction of this antenna is very well documented at http://www.njqrp.org/pac-12/. The hardest part for me was threading the aluminum rods. My homebrew PAC-12 is not perfectly straight, but it still works quite well. There is a newer design that is sold as a kit (http://www.pacificantenna.com/), and you might want to incorporate some of the new ideas if you're going to DIY.