To start right from the middle: I don't like clipless system. The fact that I broke my arm in a low-speed fall because I failed to unclip may have to do something with this position, but it's not the only reason.
Efficiency. I heard claims that clipless system greatly improves cycling efficiency - there were numbers as high as 20%. I was skeptical, so I did my own measurements in a real-world road-cycling circumstances. I was more then disappointed. I measured average speed on two loops of fixed length (60 km and 85 km) with and without clipless system, all the other factors being the same (i.e. the same bike and clothes). The average difference (averaged over 2 and 6 rides) in average speed was less then 0,5%. On one of the sections it was in favor of clipless system, on the other section it was in favor of plain pedals and shoes. The difference was smaller than the difference in individual rides within one system. Thus, my measurements doesn't show any statistically significant advantage of any of the system.
Safety. All clipless advocates confess that falling off is part of the learning process. What they do not tell is that you will damage your bike in that process and that an injury is a real possibility. My quick search through the net revealed only 4 voluntarily reported injuries caused by clipless system. When talking to the people after my accident I got the impression that they are much more common - even my physiotherapist had broken the arm. There seems to be a high degree of self-censorship, self-guilt (e.g. "it's MY fault that I couldn't unclip") and the dread of being scoffed at by the "pro"'s.
The feel. Nothing is life is all bad or all good. I liked some aspects of clipless system, especially "the feel" and "the look". I suspect that these are the main reasons for its popularity. As for me, I am not going to risk another 2 or 3 months of rehabilitation, just to stay within the "cycling mainstream". The proper cycling technique and a positive feel that your foot is not slipping on a pedal can be trained and learned, without any disadvantages of restrained feet.
Every so often I read and hear people comlaining about some additional aspects related to the clipless system: pain in feet, ankles or knees due to restricted movement, water seeping in through the holes in the soles, cold sensation due to heat transfer through the cleats, sorenes in the feet due to pressure in the cleat area, awkward walking, the need for a second footware on a tour. The general consensus, however, is that the benefits of the clipless system override any disadvantages. But what exactly are these benefits? Have they ever been proven in an authoritative research? I know of a few reasearches regarding this issue (link here) and according to them the benefits for a non-proffesional cyclist are so slim (*), that I can only conclude that clipless system is just another market gimmic with huge turnover, exploiting people's urge to follow the fashion.
(*) What this abstract says is: Shoe-pedal interface (flat pedals vs. clippless pedals) does not influence cycling effectiveness during normal (submaximal) exercise. Only an active pulling-up action on the pedal during upstroke increases the pedalling effectiveness, while reducing overall efficiency. It is, really, pretty logical: yes, when you pull up, you will be quicker, but you will loose more energy then if you didn't pull up. So, in brief, there is no free lunch; in fact, the clipless lunch is more expensive.