Here is what I wrote a while back on the subject and it is still valid for me:

Almost all the messengers and in san francisco that ride a FIXED GEAR bicycles and almost all the people that have been riding fixed for a long time on city streets ride with clips and straps. (Clipless is for road and mtn bikes!). Why? Because they know that clipless is unsafe on a fixed gear bicycle that does not have a handbrake (and even with a handbrake it is dangerous). They are unsafe because they are DESIGNED to release under certain amount of strain and when your foot moves sideways. This causes accidental unclipping when you least want it for example pulling a hockey stop at a busy intersection. With clips and straps this is not the case as long as you buy decent straps and check them every once in a while. True you can get more better power transfer with clipless shoes than with adidas sambas, but is you get some proper cycling shoes with a hard sole the difference is minimal.

It is not that clipless constantly unclip. Most of the time they are just fine. Unfortunately, they tend to unclip when you least want it - usually when putting lots of strain on them. Strain of course depends on riding style, preference of braking method, terrain, etc.
So if you go on long moderate rides in rolling country with little traffic. Clipless might never fail you.
If you enjoy going down steep hills with no hand brakes, pitching your bike sideways to scrub speed then clipless is certainly the wrong choice.

The bottom line is that clipless are designed to release when a certain angle is reached -
Clips and straps aren't. Pick what feels safer.
I have seen devout advocates of clipless unclip and it was not funny.
All of them are clip and strap riders now.

Here is the link to that painful thread:

http://www.bikeforums.net/showthread...light=clipless