I tried polyphasic sleep a few times (3 times, to be precise) recently, from December through February. A couple of people have asked how well it worked for me, and what my reactions were. Recently, I wrote this up for someone in email, and I thought for future reference I could post it here also.
The main problem was that I found it too difficult to keep the naps consistently enough to make the system work. I did reach a pretty good equilibrium at around 4.5 hours of sleep per day, on the core sleep schedule of 3 hours of sleep in the early AM plus four additional 20-25 minute naps. The problem was that I had to be disciplined about taking the naps, even if I didn't feel tired and even if I was very busy. The thing was, I would frequently find myself working on something and unable to drop it in order to go take the nap. Or I might have somewhere to be after work (at, say, 7pm) and I couldn't get away from work until 6:30, which didn't leave enough time to both nap and get to the destination. It really amounted to a discipline problem. And once I messed up the nap schedule on a given day, I would have severe tiredness problems when it came time for the core sleep, and I'd have to reset. The basic problem is that I like to be spontaneous and flexible, and polyphasic sleep isn't very amenable to that.
I could have tightened up my discipline and kept at it anyway, but I found out something interesting during the time that I did the 4.5-hours-per-day schedule. I didn't really find my life enhanced by having extra waking time. As a result, I learned something (a pretty valuable thing actually) -- that there will always be more things to do than you have time to do it in, whether you have 18 waking hours or 23. So having enough time is about *prioritizing*, not necessarily about somehow gaining more raw time to work with. Moreover, I found that when I did have extra time, I wasn't able to use it however I liked. I couldn't really read in the early morning hours; I found it too easy to fall asleep. (What I really wanted the extra time for was to get more reading done.) Also, I found that I couldn't keep going and going for as much waking time as I had; I needed a significant amount of downtime (almost as if there is a fixed amount of attention I can use in a given 24 hour period, regardless of how much time I spend awake). So I decided that I might as well be sleeping during the time that can't be otherwise useful, since I enjoy it so much anyway.