Not sure if the M520's still come with black plastic reflectors that you can unscrew, as that gives you the option of one flat side that can still be ridden with normal trainers etc, and is also ideal whilst you are still getting used to them, but can later be removed to make them double sided.
I disagree with the idea of there being no benefit to cleats if you happen to need to stop-start a lot. It is a far more effective way to put power through your peddles.
I've ridden without cleats a few times since switching (I rode for over 2 years on flat peddles before that) and it was horrible. I think we'll just have to agree to disagree
I'll just echo what others have said, I've only ever used an SPD setup with the dhb shoes and the lifeline spd compatible pedals, and I've got no complaints. I've heard some people tell me that SPD is terrible but I don't really have anything compare it to, it's fine and pretty easy to get used to.
I don't know about power efficiency or anything, I'm sure if you had cycling shoes in toe clips it'd be more or less the same. Although it's a lot harder to get in and out of toe clips. But the advantage for me wasn't anything to do with efficiency, it was that before then I was cycling in running shoes which was an absolutely abysmal experience over about 30kms, and that cleats let you to adjust the position of your foot reliably, which allowed me to eliminate some knee pain I was getting on longer rides or up climbs.
M520 - can use cleats or normal. That's what I got the first time I ventured into cleats because if it didn't work I would always change without hassle.
A520 would be better suited for that. It's mentioned above about the platforms with the M520s but I feel like that must be old M520s as myself and Nursie didn't get them last year with M520s.
Do you mean A530s? M520s can be adapted to be used with flats but I doubt it would last. A520s are single sided SPD pedals. A530s have a big flat on the other side. M324s are also combo pedals if memory serves.