Speedplay

The road pedals are sometimes called "lollipops", because, well, they look like lollipops. Advantages are that they are light, have high cornering clearance, have good float, can be engaged from either side and most people find them relatively easy to engage and disengage. Disadvantages are that some people have reported getting hot foot with these pedals.

The mountain bike pedals are also double sided, but look like a solid plastic oval with a metal ring in the middle. Advantages are that they don't get clogged with mud, are double sided, and they are mechanically very simple. Disadvantages are that some people report false "lock ins", and the cleats are not as durable (and relatively expensive).

Speedplay acheive their remarkable weight claims and high ground clearance by building their pedals the opposite way around to most other manufacturers (Look and Shimano come to mind). Speedplay pedals have the main pedalling platform built into the cleat that attaches to the shoe. Other manufacturers prefer to build smaller, lighter and cheaper cleats, and put most of the structure into the pedal.

And because you sooner or later have to stand on any cleat (even if it's only at traffic lights or before a start gun), speedplay aren't hugely popular among those who can't afford to constantly replace their equipment.

That said, there is a fairly large demand for them among those who can - they have a phenomenal amount of float. This makes adjusting them perfectly less of an issue, and means they're much kinder to those with sensitive knees then competing brands.