Metolius Astro Nuts are the top performer in parallel sided cracks. They use a unique double curve design to nearly always provide three points of contact and thus greater stability. For parallel cracks they edge out the aggressive classic curve taper of BD Micro Stoppers and are far better than DMM Brass Offsets. The vertical curve on two faces of Astro Nuts gives two good orientations for placement.
Astros are the top performer in highly featured and irregular rock. Their double curve design allows them to seat effectively around large crystals. The vertical taper is used in all but the smallest sizes as opposed to DMM Brass Offsets, which only feature cut out pods in their two largest sizes. Astros' heads are surprisingly tough and are about as durable as BD's. They are slightly offset when placing them along their more common axis, making them suitable (but not exceptional) for some pin scars. In flairs the Astros aren't nearly as good as DMM Brass Offsets, but they are far better than Micro Stoppers.
Astros use a hard metal that doesn't bite and deform into rock as well as that of DMM Offsets. Astros also don't fit pin scars or flaring pods exceptionally well. They are better than BD Micro Stoppers in pin scars but not nearly as good as DMM Brass Offsets or the DMM Peenut. They are fairly difficult to clean about as difficult as DMM Brass Offsets and slightly more fickle than Black Diamond Micro Stoppers.
Astros are available in five sizes that cover a size range equal to the six pieces of Black Diamond Micro Stopper. As a plus, this allow you to carry less gear. As a minus, there is not as much size overlap. Size overlap is more important in micro nuts because there is much less difference between a fantastic placement and a mediocre placement. This is in contrast with larger nuts, where there is more surface area and overlap is not such a big deal. Finally, Astros' cables kink pretty fast. They are a hair more durable than DMM Brass Offsets but worse than BD Micro Stoppers.
Astros are a superb choice for free climbers who like to carry a couple of brass nuts on their racks. Astros work well in all types of rock and excel in highly featured and irregular rock in areas such as Smith Rocks and The Needles of South Dakota. They fit flaring pods and pin scars fairly well but not nearly as well as the DMM Brass offsets. For aid climbing, we wouldn't carry the Astro nuts exclusively. However, they do mix well with the DMM Brass Offsets, fitting some cracks that DMM Brass Offsets don't.
Nealy all the micro nuts we looked at were around the same price per nut: $14-$15. Astros are technically cheaper per set than BDs because you only need five to cover the same range that BD covers with six. That said, you get less overlap.