I have ridden Speedplay pedals for 20 years or so. Compared to the SPD pedals I rode only a couple of times, they are by far easier to clip in and out of. On group rides, when we start from a stop it takes a few seconds for everyone else to clip in and get going. It is worth mentioning that with Speedplays you don't actually clip in, you simply pedal (they are double sided) and the pedals engage. So I am always sitting up ready to go waiting for the group to clip in and get going.
Downsides: Cost and walkability.