I know this is an old thread but I wanted to add my own experience to this in the hope it helps someone else out as well. I had a 5" clay drain pipe that I cracked into (2 pipes actually) as I was digging a trench for buried electric wire. I googled and read many posts on how to do it. It pretty much came down to either a sawzall or a snap cutter (also known as a chain cutter). I can tell you after having tried both methods the snap cutter or chain cutter is the way to go HANDS DOWN. They are pretty expensive to purchase, but I was able to rent one from my local Home Depot. They didn't think they had anything like that at first and the guy finally said "well I have something that cuts iron pipe" which was in fact the correct tool. The photo presented earlier in this post is the right tool to use. I also decided to buy a diamond tip sawzall blade (works like a carbide blade only lasts longer) just in case. Let me tell you the snap cutter once I got it in place and figured out how it worked took me less than 30 seconds to cut the pipe. I did end up having to use the sawzall because the first end that I cut had a crack underneath that I could not see and I ended up with an uneven cut (not terrible, but you want as clean a cut as you can so your PVC pipe can fit right up to it) I used the sawzall with the diamond tipped blade and while that worked as well it still took me about a minute to cut through 2 inches of the pipe. If I had used the sawzall on the entire pipe I think I would have been a few minutes at it at least.
I think you could use a sawzall but even those blades aren't going to last forever and my one diamond-tipped blade cost me $15 which was interestingly enough also the rental cost for the snap tool.
So, the snap/chain cutter in my opinion is the way to go if you can get your hands on one to rent. It definately is a a case-in-point for the old addage "the right tool for the job makes all the difference"