I use Speedplay Zeros on my road, commuting fixed and track bikes. They're the only pedals I've used for the last five years and I'm really very happy with them. The downsides that I have experienced are that the cleats can require a lot of force to engage when brand new, apparently this is an issue for lighter riders (which is addressed by the 'light action' pedal); they can get a bit sticky if contaminated with grit; the cleats are expensive (but pretty hard wearing); there is not a large range of four hole shoes optimised for Speedplay (but the adapters add very little weight or height to the shoe).
Highlights are the adjustability, ease of maintenance, easy double sided entry, neatness, and retention - I don't think I have ever pulled a foot out, and I pull up fairly hard.