I have clipless SPDs on one road bike, old fashioned clips and straps (but don't use cleats) on two other road bikes, and flats with grip pins on my mountain bike. Frankly, the only advantage of either the SPDs or the clips/straps over the flats is that they keep my foot in position without thinking about it. I like both SPDs and clips/straps, but IMHO (and based on several published studies) they have zero power advantage, as mentioned by a couple of other folks above. I seriously don't understand non-racers who use non-SPD clipless pedals. The weight savings is minimal and the PITA factor for walking around at your destination is ridiculous.
Oddly, some folks love clipless pedals but hate clips/straps. For myself, I don't perceive much difference between them as long as you don't tighten up the straps too much or use old-fashioned cleats which trap you in the pedal until you loosen the straps.
For the longest time I used SPDs on my mountain bike too, but this last year I rode some extremely rocky steep trails and fell off several times due to having insufficient time to twist out. That made me switch to flats with grip pins. I've also noticed that many of the mountain bikers out there doing rocky trails (including racers) are doing the same these days whereas previously everybody was on SPDs. I am SOOO much happier with the flats and perceive zero disadvantages.