I have Shimano PD-A530 on my commuter bike. I use the SPD side 90% of the time. I like them. I keep a pair of tennis shoes at work to change into when I arrive.

The only times I use the flat side are when 1) I'm towing my daughter on her tag along, 2) Short trip to a store, or 3) I'm meeting someone after work and I don't want to carry a second pair of shoes in my bag. It's nice to have the option to ride with regular shoes and no retention.

The flat sides of the pedals are not particularly grippy, but they work well enough. If I'm riding with "regular" shoes I consider it a take-it-easy day.

I've never used any of the pinned, mountain bike, platform pedals. I don't know if I would like them or not. I know I like being attached with the SPDs. For me it's not so much a matter of efficiency or getting more power to the crank. I just feel more secure with good foot retention.