The big thing about Speedplays (and also much older 'Time' cleats and pedals) is the sheer amount of float, which is great for dodgy knees. Also being double sided means much easier entry what setting off from traffic lights, etc.
With Speedplay the pedals are cheap (relatively) but the cleats are expensive but last a long time and offer a lot of adjustability.
Personally, I'd struggle to go back to SPD-SL or Look cleats.
The big thing about Speedplays (and also much older 'Time' cleats and pedals) is the sheer amount of float, which is great for dodgy knees. Also being double sided means much easier entry what setting off from traffic lights, etc.
With Speedplay the pedals are cheap (relatively) but the cleats are expensive but last a long time and offer a lot of adjustability.
Personally, I'd struggle to go back to SPD-SL or Look cleats.